This issue (mixup) has been widely reported, it's strictly related to flash and nvidia. You can search the forum around "ghost images" for further info. The issue come and go depending on different flash-player-plugin versions and/or different nvidia driver versions, and different Xserver version. (and of course depending on video card too).
Turning off acceleration should solve also this issue, although occasionally I see some artifacts, mostly is solved for me.
Glad you solved

AS