Modern sports are constantly fighting against attempts to cheat. What sort of cheating went on at tournaments? Successful or unsuccessful, common, or one-shot, what was done to throw the match, take a dive, use performance-enhancing lances, or otherwise win unfairly? Thanks!
Not strictly cheating but more than a couple of treatises have underhanded and unconventional tactics for the battlefield.
The two that come to mind is the caustic powder you can hide inside the head of a hollow axe-head. More about can be read on this blog. The other technique was for wrestling in armour; whilst on the ground, you rip either your or their tabbard to shreds and stuff them into your opponent's visor - blinding them to your dagger or empty hand strikes.