This is the guy who we're told is responsible for the biggest screwup in Oscar history ... the guy who handed Warren Beatty the wrong envelope. His name -- Brian Cullinan -- an accountant at Pricewaterhouse. He's a veteran ... 30 years at the accounting firm. Here's the thing ... Brian was supposed to discard the spare Best Actress envelope and hand Warren Best Picture. As for why ... we know this. Brian was tweeting like crazy during the ceremony, posting photos ... so he may have been distracted. Brian has since deleted the tweets, but one was a pic...