I have found some threads about the Steam overlay + UltimateFullscreen that was causing trouble to some developers, but those threads don't have a solution for this.
We have a game soon to be released on Steam, and we are using Fusion and a custom CRT shader for the game.
Our last game "Oniken" was supposed to be released with this shader effect, but we couldn't fix this issue earlier and then Oniken was released without the CRT shader.
Now we need to release this new game with the shader. Everything is fine when you test the game .EXE standalone, but when we launch the game trough Steam, the overlay shows up, and quickly after the first frame when our team logo appears and the frame changes, the screen freezes and the game continue to run in background (you can hear the sounds and even play the game, but with the screen frozen at the first frame).
There is a fix for this? I was suspecting it was Ultimate Fullscreen object, but we had removed it completely and the problem still persists...
Our shader in action: http://imgur.com/4z6hilF