Comment 7 for bug 978430

You'd need to press enter with Onboard to reproduce this. When enter is clicked, Onboard fakes pressing enter down, then gets killed before it can send the key-release. X thinks the key is still pressed down, key repeat sets it and everything is spammed with n enter presses per second, wreaking all kinds of havoc on the desktop. The effect is the same as if you forever held down the enter key on you physical keyboard.
It might not always be reproducible, though. If the greeter takes just long enough to kill Onboard everything may be fine. I've seen it happen on Quantal not long ago though.
Thanks for looking at those bug reports btw. :)