I got into an inverted spin in a JP5 (hard to imagine how you could do it by accident - but we did!). It felt like an erect spin, but was very nose down and very high rotational - and very disorientating. It was only the T+S which was telling us it that we were inverted that it became clear what we had to do to recover. However, we were both reluctant to believe it. The recovery was immediate when it was initiated, but it took us so long to think about what was going on and what to do about it, we bottomed out below ejection height.
We snagged the aircraft. It turned out one of the wings as out of incidence following an overstress.