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 of Emma Stone with her Oscar.

If it was tweeted right after her win, that's exactly when Brian should've been prepping for the next category ... Best Picture.

No word on whether discipline is in store.

We're told the firm is issuing an apology to all concerned.