I don't know that I've ever had anything terrible, but I did once get some pasta from Boardwalk Pizza and Pasta that was incredibly mediocre. I wasn't exactly thrilled with it, but I was starving and it was the only place that was practically empty, so I went for it.
I don't know if they really count as worst but I am not a fan of the Mickey beignets and as much as I wanted to like the Cheesy Garlic Pretzel bread I found it very greasy.