Some years ago I experienced a C150 which was unrecoverable in the spin using Standard Spin Recovery technique. In the end I used in-spin aileron and recoverd at 1100 feet having been well over 4300 when I entered the spin. I was training an Instructor for aerobatics using his club's aircraft. The aircraft was subsequently grounded following my report to the CAA and it transpired that it was badly rigged and had been so for a loong time, despite being certified by the engineers as airworthy. A potential killer due to the fact that it is easy to be complacent about Cessnas as the myth states they recover so easily. One can easily be lulled into a false sense of security by assuming that the maintenance docs must be correct if you are flying an FTO's aircraft. I learnt a lot from this episode and would counsel against ever commencing a spin below 4000 feet, despite the fact that our H o T regularly does spins in C152s below 2400 during instrucotr tests - would not get me doing it though...