It doesn't happen all the time but very frequently that my window loose it's top-border.
When I tab between windows, or just click between them, the border of the previous window is lost. Can get it back if I just tab again between the windows.
It's been like this all the time in Ubuntu 8.04.
My system is:
AMD Opteron 165
GF 6800GT with the EnvyNG-driver
This is what it looks like: