Had the ref thrown the flag, they could have reviewed the play to see if it was tipped. If it was, they could pick up the flag. But since there wasn't a flag, they didn't have the chance to go to review.

Disclaimer - I am a Rams fan. I am in stunned disbelief that there wasn't a flag on that call. They could have called either the PI or the helmet to helmet. I 100% agree that the Rams got away with one on that call. With that said, the Saints D should have been able to stop the Rams from driving for the FG to tie in regulation. They shouldn't have given up the INT in OT. If the Saints receiver plays the ball instead of running into his defender trying to draw a PI in that case, he may have caught the ball, or at least kept the Rams defender from getting the INT. The Saints still had chances to win. But, with all of that said, the Rams shouldn't have had a chance in regulation to tie had the refs gotten that call correct. The refs extended that game, and the Rams took advantage at that point.