This is what the PrimeGrid errors look like. I just got the same set of errors in Collatz. I'm copying the Collatz message here for informational purposes and will also post them in Collatz.
Thu Apr 26 19:04:39 2012 | Collatz Conjecture | Task collatz_2375033642863672666472_824633720832_1: no shared memory segment
Thu Apr 26 19:04:39 2012 | Collatz Conjecture | Task collatz_2375033642863672666472_824633720832_1 exited with zero status but no 'finished' file
Thu Apr 26 19:04:39 2012 | Collatz Conjecture | If this happens repeatedly you may need to reset the project.
I think it may represent the GPU ran out of memory and silently stopped working until it was told to suspend and then BOINC notices it has stopped and posts the error.