diff --git a/lib/i18n/backend/lazy_loadable.rb b/lib/i18n/backend/lazy_loadable.rb
index f519960f..92ee8415 100644
--- a/lib/i18n/backend/lazy_loadable.rb
+++ b/lib/i18n/backend/lazy_loadable.rb
@@ -36,7 +36,7 @@ module Backend
     #
     # The backend has two working modes: lazy_load and eager_load.
     #
-    # We recommend enabling this to true in test environments only.
+    # Note: This backend should only be enabled in in test environments!
     # When the mode is set to false, the backend behaves exactly like the
     # Simple backend, with an additional check that the paths being loaded
     # abide by the format. If paths can't be matched to the format, an error is raised.
@@ -48,7 +48,7 @@ module Backend
     #
     #   I18n.backend = I18n::Backend::LazyLoadable.new(lazy_load: true)
     #
-    #   # In other environments, such as Prod and CI
+    #   # In other environments, such as production and CI
     #
     #   I18n.backend = I18n::Backend::LazyLoadable.new(lazy_load: false) # default
     #
@@ -95,16 +95,6 @@ def eager_load!
         end
       end
 
-      # Select all files from I18n load path that belong to current locale.
-      # These files must start with the locale identifier (ie. "en", "pt-BR"),
-      # followed by an "_" demarcation to separate proceeding text.
-      def filenames_for_current_locale
-        I18n.load_path.flatten.select do |path|
-          LocaleExtractor.locale_from_path(path) == I18n.locale &&
-          supported_extension?(path)
-        end
-      end
-
       # Parse the load path and extract all locales.
       def available_locales
         if lazy_load?
@@ -116,16 +106,18 @@ def available_locales
 
       protected
 
+
       # Load translations from files that belong to the current locale.
       def init_translations
-        if lazy_load?
-          load_translations(filenames_for_current_locale)
+        file_errors = if lazy_load?
           initialized_locales[I18n.locale] = true
+          load_translations_and_collect_file_errors(filenames_for_current_locale)
         else
-          super
-          filenames_named_incorrectly = I18n.load_path.reject { |path| file_named_correctly?(path) }
-          raise InvalidFilenames.new(filenames_named_incorrectly) unless filenames_named_incorrectly.empty?
+          @initialized = true
+          load_translations_and_collect_file_errors(I18n.load_path)
         end
+
+        raise InvalidFilenames.new(file_errors) unless file_errors.empty?
       end
 
       def initialized_locales
@@ -138,15 +130,44 @@ def lazy_load?
         @lazy_load
       end
 
-      SUPPORTED_EXTENSIONS = [".yml", ".yaml", ".po", ".json", ".rb"].freeze
+      class FilenameIncorrect < StandardError
+        def initialize(file, expected_locale, unexpected_locales)
+          super "#{file} can only load translations for \"#{expected_locale}\". Found translations for: #{unexpected_locales}."
+        end
+      end
+
+      # Loads each file supplied and asserts that the file only loads
+      # translations as expected by the name. The method returns a list of
+      # errors corresponding to offending files.
+      def load_translations_and_collect_file_errors(files)
+        errors = []
+
+        load_translations(files) do |file, loaded_translations|
+          assert_file_named_correctly!(file, loaded_translations)
+        rescue FilenameIncorrect => e
+          errors << e
+        end
 
-      def supported_extension?(path)
-        path.end_with?(*SUPPORTED_EXTENSIONS)
+        errors
       end
 
-      def file_named_correctly?(path)
-        extracted_locale = LocaleExtractor.locale_from_path(path)
-        available_locales.include?(extracted_locale)
+      # Select all files from I18n load path that belong to current locale.
+      # These files must start with the locale identifier (ie. "en", "pt-BR"),
+      # followed by an "_" demarcation to separate proceeding text.
+      def filenames_for_current_locale
+        I18n.load_path.flatten.select do |path|
+          LocaleExtractor.locale_from_path(path) == I18n.locale
+        end
+      end
+
+      # Checks if a filename is named in correspondence to the translations it loaded.
+      # The locale extracted from the path must be the single locale loaded in the translations.
+      def assert_file_named_correctly!(file, translations)
+        loaded_locales = translations.keys.map(&:to_sym)
+        expected_locale = LocaleExtractor.locale_from_path(file)
+        unexpected_locales = loaded_locales.reject { |locale| locale == expected_locale }
+
+        raise FilenameIncorrect.new(file, expected_locale, unexpected_locales) unless unexpected_locales.empty?
       end
     end
   end
diff --git a/lib/i18n/exceptions.rb b/lib/i18n/exceptions.rb
index c4c69093..05741633 100644
--- a/lib/i18n/exceptions.rb
+++ b/lib/i18n/exceptions.rb
@@ -121,10 +121,26 @@ def initialize(method, backend_klass)
   end
 
   class InvalidFilenames < ArgumentError
-    attr_reader :invalid_filenames
-
-    def initialize(invalid_filenames)
-      super "Locales cannot be extracted from the following paths: #{invalid_filenames}"
+    NUMBER_OF_ERRORS_SHOWN = 20
+    def initialize(file_errors)
+      super <<~MSG
+        Found #{file_errors.count} error(s).
+        The first #{[file_errors.count, NUMBER_OF_ERRORS_SHOWN].min} error(s):
+        #{file_errors.map(&:message).first(NUMBER_OF_ERRORS_SHOWN).join("\n")}
+
+        To use the LazyLoadable backend:
+        1. Filenames must start with the locale.
+        2. An underscore must separate the locale with any optional text that follows.
+        3. The file must only contain translation data for the single locale.
+
+        Example:
+        "/config/locales/fr.yml" which contains:
+        ```yml
+          fr:
+            dog:
+              chien
+        ```
+      MSG
     end
   end
 end
diff --git a/lib/i18n/tests/basics.rb b/lib/i18n/tests/basics.rb
index cc9d162b..be824302 100644
--- a/lib/i18n/tests/basics.rb
+++ b/lib/i18n/tests/basics.rb
@@ -9,7 +9,7 @@ def teardown
         I18n.backend.store_translations('de', :foo => 'bar')
         I18n.backend.store_translations('en', :foo => 'foo')
 
-        assert_equal I18n.backend.available_locales, I18n.available_locales
+        assert_equal I18n.available_locales, I18n.backend.available_locales
       end
 
       test "available_locales can be set to something else independently from the actual locale data" do
@@ -23,7 +23,7 @@ def teardown
         assert_equal [:foo, :bar], I18n.available_locales
 
         I18n.available_locales = nil
-        assert_equal I18n.backend.available_locales, I18n.available_locales
+        assert_equal I18n.available_locales, I18n.backend.available_locales
       end
 
       test "available_locales memoizes when set explicitely" do
diff --git a/test/backend/lazy_loadable_test.rb b/test/backend/lazy_loadable_test.rb
index 50076c66..968efe41 100644
--- a/test/backend/lazy_loadable_test.rb
+++ b/test/backend/lazy_loadable_test.rb
@@ -73,13 +73,12 @@ def setup
     end
   end
 
-  test "lazy mode: loads translations from files that start with current locale identifier or contain identifier in path components, and end with a supported extension" do
+  test "lazy mode: loads translations from files that start with current locale identifier" do
     with_lazy_mode do
       file_contents = { en: { alice: "bob" } }.to_yaml
 
       invalid_files = [
         { filename: ['translation', '.yml'] },                # No locale identifier
-        { filename: ['en_translation', '.unsupported'] },     # Unsupported extension
         { filename: ['translation', '.unsupported'] },        # No locale identifier and unsupported extension
       ]
 
@@ -104,6 +103,17 @@ def setup
     end
   end
 
+  test "lazy mode: files with unsupported extensions raise UnknownFileType error" do
+    with_lazy_mode do
+      file_contents = { en: { alice: "bob" } }.to_yaml
+      filename =  ['en_translation', '.unsupported']     # Correct locale identifier, but unsupported extension
+
+      with_translation_file_in_load_path(filename, nil, file_contents) do
+        assert_raises(I18n::UnknownFileType) { I18n.t("foo.bar") }
+      end
+    end
+  end
+
   test "lazy mode: #available_locales returns all locales available from load path irrespective of current locale" do
     with_lazy_mode do
       I18n.with_locale(:en) { assert_equal [:en, :fr], @backend.available_locales }
@@ -111,6 +121,34 @@ def setup
     end
   end
 
+  test "lazy mode: raises error if translations loaded don't correspond to locale extracted from filename" do
+    filename = ["en_", ".yml"]
+    file_contents = { fr: { dog: "chien" } }.to_yaml
+
+    with_lazy_mode do
+      with_translation_file_in_load_path(filename, nil, file_contents) do |file_path|
+        exception = assert_raises(I18n::InvalidFilenames) { I18n.t("foo.bar") }
+
+        expected_message = /#{Regexp.escape(file_path)} can only load translations for "en"\. Found translations for: \[\:fr\]/
+        assert_match expected_message, exception.message
+      end
+    end
+  end
+
+  test "lazy mode: raises error if translations for more than one locale are loaded from a single file" do
+    filename = ["en_", ".yml"]
+    file_contents = { en: { alice: "bob" },  fr: { dog: "chien" }, de: { cat: 'katze' } }.to_yaml
+
+    with_lazy_mode do
+      with_translation_file_in_load_path(filename, nil, file_contents) do |file_path|
+        exception = assert_raises(I18n::InvalidFilenames) { I18n.t("foo.bar") }
+
+        expected_message = /#{Regexp.escape(file_path)} can only load translations for "en"\. Found translations for: \[\:fr\, \:de\]/
+        assert_match expected_message, exception.message
+      end
+    end
+  end
+
   test "eager mode: load all translations, irrespective of locale" do
     with_eager_mode do
       @backend.reload!
@@ -128,9 +166,11 @@ def setup
 
       contents = { de: { cat: 'katze' } }.to_yaml
 
-      with_translation_file_in_load_path(['translation', '.yml'], nil, contents) do |file_path|
+      with_translation_file_in_load_path(['fr_translation', '.yml'], nil, contents) do |file_path|
         exception = assert_raises(I18n::InvalidFilenames) { I18n.t("foo.bar") }
-        assert_equal "Locales cannot be extracted from the following paths: #{[file_path]}", exception.message
+
+        expected_message = /#{Regexp.escape(file_path)} can only load translations for "fr"\. Found translations for: \[\:de\]/
+        assert_match expected_message, exception.message
       end
     end
   end