Yesterday I just closed my mbp and this morning I had the great pleasure to have this:

At the login (passwork) screen...
I had an uptime of more than 5 days but still ... I upgraded the memory from 4GB to 8BG (the Crucial kit) and since then I had this problem twice. Mayve it doesn't mean anything because I have the mpb for only 2 weeks and I upgraded the memory after a few days, so the statistics are maybe meaningless.
My question is: do you have this problem sometimes ? Should I check something in particular to try to avoid that ? Is it possible that the memory is incorrectly installed (very unlikely...) ? Is it in any case a memory problem ?
