Well I was in a large-small(early game) battle with Venice on a bridge scenario.
As the title says, I had 2 pairs of balista and I decided to use it against the Italians sitting dumbly accross the bridge.

But to get within range, I had to cross about 1/4 of the bridge. No problem, the Venetians didn't seem to react at all. But then I noticed my balista crew dying. The venetian archers were idle. I panned to my balista and saw the crew all hugging the side of the bridge and then disappearing 1 by 1, not falling, just disappearing.
I tried to move the balista so that the crew would stay in the middle, but as all narrow bridge battles, its virtually impossible to navigate and my seige engine ultimate ends up disappearing as well.

I believe its a collision issue on the bridge but anyway this bug cost me many men. Unfortunately, I forgot to take any screenies so just try to imagine it.