i had that same problem, i forgot to hand in my I-94 card or which ever card it was the white one with my J-1, took a 2 week visit to Canada and when I came back to the US the guy was like why do you still have this, I think in the end he realized it was an honest mistake and he took it off me while I filled out another one as I had a day left before my 30 day grace period on my J-1