Well I doubt that I am the only person to have experienced trying to demonstrate spinning and having the aircraft NOT recover despite using the "standard spin recovery" method. Aircraft in question was a C150 aerobat and it did not respond at all to the usual method. In the end I had to use in-spin aileron to recover having lost well in excess of 3000 feet. Bottomed out at just 1100 feet.
Once safely back on the ground, the aircraft was stripped down by the engineers who found nothing wrong, however, the CAA test pilots flew it and grounded it and when it was examined properly it turned out to have a bent wing which was several degrees out of true.
The lesson I learnt was never to trust the paperwork of engineers and a flight school you do not know (I was a guest FIC instructor there).