diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index 712ad8cef9..261d37d4ee 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -7,10 +7,16 @@ package runtime
 // This file contains the implementation of Go channels.
 
 // Invariants:
-//  At least one of c.sendq and c.recvq is empty.
+//  At least one of c.sendq and c.recvq is empty,
+//  except for the case of an unbuffered channel with a single goroutine
+//  blocked on it for both sending and receiving using a select statement,
+//  in which case the length of c.sendq and c.recvq is limited only by the
+//  size of the select statement.
+//
 // For buffered channels, also:
 //  c.qcount > 0 implies that c.recvq is empty.
 //  c.qcount < c.dataqsiz implies that c.sendq is empty.
+
 import (
 	"runtime/internal/atomic"
 	"unsafe"