diff --git a/prometheus/histogram.go b/prometheus/histogram.go
index 3346fa1c5..8425640b3 100644
--- a/prometheus/histogram.go
+++ b/prometheus/histogram.go
@@ -47,7 +47,12 @@ type Histogram interface {
 	Metric
 	Collector
 
-	// Observe adds a single observation to the histogram.
+	// Observe adds a single observation to the histogram. Observations are
+	// usually positive or zero. Negative observations are accepted but
+	// prevent current versions of Prometheus from properly detecting
+	// counter resets in the sum of observations. See
+	// https://prometheus.io/docs/practices/histograms/#count-and-sum-of-observations
+	// for details.
 	Observe(float64)
 }
 
diff --git a/prometheus/summary.go b/prometheus/summary.go
index 50e48bad8..c5fa8ed7c 100644
--- a/prometheus/summary.go
+++ b/prometheus/summary.go
@@ -55,7 +55,12 @@ type Summary interface {
 	Metric
 	Collector
 
-	// Observe adds a single observation to the summary.
+	// Observe adds a single observation to the summary. Observations are
+	// usually positive or zero. Negative observations are accepted but
+	// prevent current versions of Prometheus from properly detecting
+	// counter resets in the sum of observations. See
+	// https://prometheus.io/docs/practices/histograms/#count-and-sum-of-observations
+	// for details.
 	Observe(float64)
 }