Training past trainer's level
 

At least on make poison if you keep clicking the NPC will say he can't possibly teach you any more, yet your skill level keeps going up (and your plat keeps disappearing) but when you zone it all seems to reset. Skill goes back down, plat returns.

I can't believe I didn't notice it last time I "trained" up but today I was wondering where all my levels in skills went. It doesn't seem to eat your money but I honestly didn't keep track the first time. Only tried this on make poison skill.