diff --git a/src/os/file_plan9.go b/src/os/file_plan9.go
index 74c377127d..e4f8fd987b 100644
--- a/src/os/file_plan9.go
+++ b/src/os/file_plan9.go
@@ -29,6 +29,7 @@ type file struct {
 
 // Fd returns the integer Plan 9 file descriptor referencing the open file.
 // The file descriptor is valid only until f.Close is called or f is garbage collected.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))
diff --git a/src/os/file_unix.go b/src/os/file_unix.go
index 84a2bb5f00..b834f52589 100644
--- a/src/os/file_unix.go
+++ b/src/os/file_unix.go
@@ -54,6 +54,7 @@ type file struct {
 
 // Fd returns the integer Unix file descriptor referencing the open file.
 // The file descriptor is valid only until f.Close is called or f is garbage collected.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))
diff --git a/src/os/file_windows.go b/src/os/file_windows.go
index c8307a6d22..81a0ab7346 100644
--- a/src/os/file_windows.go
+++ b/src/os/file_windows.go
@@ -25,6 +25,7 @@ type file struct {
 
 // Fd returns the Windows handle referencing the open file.
 // The handle is valid only until f.Close is called or f is garbage collected.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 func (file *File) Fd() uintptr {
 	if file == nil {
 		return uintptr(syscall.InvalidHandle)