Hi!

Ive found that (for me) this layout is causing the error:
Section "ServerLayout"
  Identifier     "My X Server"
  Screen      0  "LeftScreen" 0 0
  Screen      1  "RightScreen" RightOf "LeftScreen"
    InputDevice "Mouse1" "CorePointer"
    InputDevice "Keyboard1" "CoreKeyboard"
EndSection

If I remove "Screen 1" then the error dont occur... Both devices are using
radeon driver...

I cannot test if the error occur only when both devices are radeon, or if
having two Screens of any type...

I think the destroy buffer is called twice because of two screens...

Anyway, the workaround of checking "valid" value fixes this issue for me...

Sorry, I cant test vanilla, I am using Slackware64... Ive downloaded the source
DVD and changed x11.SlackBuild... Only changed CFLAGS to -O0 -g3 and removed
"strip" symbols...

Thanks...
Weber Kai