Some of the dumbest ones that I've heard are actually quite common practice:
*Prune your roots every week ; cutting all the tips off helps them up-take nutrients faster.
*Hanging the plant upside down makes all the THC flow to the outside and increase the number of crystals.
*Piss on your plants to give them extra N and make them grow faster.
*Flush with vanilla extract (or fruit juice or soda or beer etc.) will give your plants extra flavor.
*Water curing. (Yes, I've tried it. It is retarded; enough said)
*Pouring boiling water on your plants will increase potency and make it cure better.
*Use nothing but
Advanced Nutrients products. (Quite possibly the dumbest advice one could give)
*Flushing and curing has no effect on the finished product.
*Using a microwave or oven to dry you buds.