Almost definitely the upper limit screw. Do like flatballer said.

However, I've had a couple cranks where setting the high limit so close that you could barely fit a hair between the FD's outer plate and the chain didn't prevent the chain from getting thrown. The cranks themselves would start to come apart (the spider was press-fitted onto the spindle and would work its way loose after a few hundred miles) and the ring assembly would wobble as if there was severe frame flex in the BB area (one bike was a CAAD8... flex?? Bah! ).

Shift into a tall gear on the big ring, stand up, and the chain gets tossed when my right foot went down. I let one bike get bad enough that I could do it at will.

Switched to another brand of crank and it never happened again.

What's on your bike?