In my Pitts, I ofern let people try some maneouvers but the ground brief states that if I donI like it I will tell them and expect to be given control - once (with a Sheffield based Ppruner) he just couldn't hear me and i ended up overpowering him on the stick. You know who you are!!
I regret not doing it on one occasion in someone else's taildragger. He was new to the aeroplane and it was too much aeroplane for him - and ended up on the runway backwards at 60kts with the aeroplane shedding large bits rapidly. Gear leg, propellor, large round engine, etc!
I just knew it was wrong on short final and tried to gently persuade him that we were not lined up, he'd failed to take into account the wind from right to left and the g/s was 50% too high. It was recoverable right to the point when with the port main on the tarmac (and I assume the brake locked on) he hauled the column back into his belly and applied full throttle - we went round like a top - both tyres now squealing over the the engine at full power until the starboard gear snapped off.........
I still have what-ifs over that one!
