diff --git a/src/black/__init__.py b/src/black/__init__.py
index 64a18655905..f0920b5a26c 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -112,6 +112,10 @@ class InvalidInput(ValueError):
     """Raised when input source code fails all parse attempts."""
 
 
+class BracketMatchError(KeyError):
+    """Raised when an opening bracket is unable to be matched to a closing bracket."""
+
+
 T = TypeVar("T")
 E = TypeVar("E", bound=Exception)
 
@@ -1308,7 +1312,13 @@ def mark(self, leaf: Leaf) -> None:
         self.maybe_decrement_after_lambda_arguments(leaf)
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
-            opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
+            try:
+                opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
+            except KeyError as e:
+                raise BracketMatchError(
+                    "Unable to match a closing bracket to the following opening"
+                    f" bracket: {leaf}"
+                ) from e
             leaf.opening_bracket = opening_bracket
             if not leaf.value:
                 self.invisible.append(leaf)
@@ -3306,10 +3316,26 @@ def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
                 yield TErr(
                     "Will not strip parentheses which have comments attached to them."
                 )
+                return
 
         new_line = line.clone()
         new_line.comments = line.comments.copy()
-        append_leaves(new_line, line, LL[: string_idx - 1])
+        try:
+            append_leaves(new_line, line, LL[: string_idx - 1])
+        except BracketMatchError as e:
+            # HACK: I believe there is currently a bug somewhere in
+            # right_hand_split() that is causing brackets to not be tracked
+            # properly by a shared BracketTracker. It is probably fine to just
+            # ignore this event using the `preformatted=True` keyword argument,
+            # but to be safe we abort this transformer when this happens.
+            cant_transform = CannotTransform(
+                "While attempting to strip parens, we were unable to match one of the"
+                " other brackets (i.e. NOT one of the target parens) in this line to"
+                " its corresponding opening/closing bracket."
+            )
+            cant_transform.__cause__ = e
+            yield Err(cant_transform)
+            return
 
         string_leaf = Leaf(token.STRING, LL[string_idx].value)
         LL[string_idx - 1].remove()
diff --git a/tests/data/long_strings__regression.py b/tests/data/long_strings__regression.py
index 044bb4a5deb..823879c729d 100644
--- a/tests/data/long_strings__regression.py
+++ b/tests/data/long_strings__regression.py
@@ -310,6 +310,25 @@ def who(self):
             passenger_association=passenger_association,
         )
 
+def A():
+    def B():
+        def C():
+            def D():
+                def E():
+                    def F():
+                        def G():
+                            assert (
+                                c_float(val[0][0] / val[0][1]).value
+                                == c_float(value[0][0] / value[0][1]).value
+                            ), "%s didn't roundtrip" % tag
+
+class xxxxxxxxxxxxxxxxxxxxx(xxxx.xxxxxxxxxxxxx):
+    def xxxxxxx_xxxxxx(xxxx):
+        assert xxxxxxx_xxxx in [
+            x.xxxxx.xxxxxx.xxxxx.xxxxxx,
+            x.xxxxx.xxxxxx.xxxxx.xxxx,
+        ], ("xxxxxxxxxxx xxxxxxx xxxx (xxxxxx xxxx) %x xxx xxxxx" % xxxxxxx_xxxx)
+
 # output
 
 
@@ -702,3 +721,26 @@ def who(self):
                 passenger_association=passenger_association,
             )
         )
+
+
+def A():
+    def B():
+        def C():
+            def D():
+                def E():
+                    def F():
+                        def G():
+                            assert (
+                                c_float(val[0][0] / val[0][1]).value
+                                == c_float(value[0][0] / value[0][1]).value
+                            ), ("%s didn't roundtrip" % tag)
+
+
+class xxxxxxxxxxxxxxxxxxxxx(xxxx.xxxxxxxxxxxxx):
+    def xxxxxxx_xxxxxx(xxxx):
+        assert xxxxxxx_xxxx in [
+            x.xxxxx.xxxxxx.xxxxx.xxxxxx,
+            x.xxxxx.xxxxxx.xxxxx.xxxx,
+        ], (
+            "xxxxxxxxxxx xxxxxxx xxxx (xxxxxx xxxx) %x xxx xxxxx" % xxxxxxx_xxxx
+        )