I bet she didn't pull the slide to the rear completly before trying to chamber the next round. what you end up with is a fired round and live round wedged inside the open action that is now locked up. The shell lifter is trying to raise the live shell but it can't because the fired shell is in the way because it was only extracted and not ejected. I know you can jam up an 870 pretty good by short shucking the slide, so its not just the mossbergs that jam. You only need to have that happen one time before you learn to pump that slide with some authority making sure it comes completely back and hits its stop before pushing it forward.