diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 9ea058db3ed..7483946fc0a 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -244,6 +244,8 @@ func (b *Reader) Read(p []byte) (n int, err error) {
 	}
 
 	// copy as much as we can
+	// Note: if the slice panics here, it is probably because
+	// the underlying reader returned a bad count. See issue 49795.
 	n = copy(p, b.buf[b.r:b.w])
 	b.r += n
 	b.lastByte = int(b.buf[b.r-1])