Open two tabs, and with the tab sidebar open, click on 1 then the other, alternating between them. Every click out of 10-20 clicks or so, clicking will *not* change the displayed tab in the main window - it will still show the previous tab (but the thumbnail will be marked as active after clicking it). So, if you click 20 times, you will likely see this at least once. This is an e10s GTK build on Ubuntu 10.04.
related to bug 552809?
flagging to block 2.0
I could not reproduce this (also using an e10s GTK build on Ubuntu 10.04).
I can no longer reproduce this.