So I've made this mistake a couple of times, I'll forget I'm in 2nd and be going at a very low speed to the point of the engine "lugging". Usually this is fine and nothing happens before I notice and downshift.

A couple of times though, something has made a bang. Both times, it has been while the clutch is partially engaged and the engine is lugging. It honestly sounds like the clutch has exploded and I fully expect it to be screwed, but it always ends up perfectly fine. That's why I'm beginning to suspect that it could be a nasty backfire, or maybe one of the engine mounts is complaining.

Any insight? Sorry for the MINI gore.