Same problem here in level 101 of Dead Man's Booty. There is something invisible and indestructible in the rightmost plank square in the 2nd row from the bottom that my plants insist on firing at ceaselessly. I can see the impacts hitting what appears to be thin air. Whatever is or is not there never dies, though, so the level never ends. Unfortunately, I am also provided merely with the options to "save and quit" or "continue", so I can't restart.
