Xorg (and whole system sometimes, e.g. alt+sysrq+reisub doesn't work) on flash video playback(youtube, vimeo, etc). Sometimes it plays normally, but freeze after closing page. I tried to install i386 and x86-64 versions of flash-plugins, play with EnableLinuxHWVideoDecode parameter in /etc/adobe/mmc.cfg, install 285 instead 280 version of nvidia drivers, and nothing helps.
Everything works well on 11.04.
I think this is a problem on the nVidia graphics card.
Full system reinstall has solved this problem for me.
Well, this is a linux system - I'm not willing to handle it as a Windows system!
This is not gnome2 problem. I had this problem in unity.
At least this fast-forward bug seems to be fixed with adobe-flashplugin 11.2.202.228-0precise1.