I have seen an intermittent, but reproducible problem.  Emacs 23 or 24 on Ubuntu 12.04 using X (nearly freshly installed).

If I type into a buffer, (including minibuf), I see some characters overwritten with a box (of background color).

Typing faster provokes the problem.  redraw-frame fixes the display.

Changing font does not help. 

No problem if “emacs –nw”, but that is pretty limiting.

Thanks.

Oz DiGennaro