Well, I would assume that yes, it is possible. It seems to me that it is during "processing" that any errors would be found, so until your status moves to "check approved", there's always the chance that an error will show up.
FWIW, I didn't send certified (wasn't aware that I should) and my status shows "processing" with "Error(s): None" as well.
I wasn't aware that there was a problem going on with the rebates. I'll have to do a search and see what stories people were sharing...