Try not to think of a white bear, and a white bear is all you'll think about
Daniel Wegner asked volunteers to avoid thinking of a white bear for five minutes, ringing a bell every time it popped into their head. Suppression backfired: the bear intruded more often than for people told to think about it freely. Wegner called it ironic process theory — mental control monitors for the banned thought, which keeps resurfacing it.