Words that are their own opposite: contronyms are chaos

A contronym is a word that means two opposite things at the same time, which should be illegal but somehow isn't. You use these constantly and never notice, because context quietly does the heavy lifting. Then someone points them out and the ground gives way beneath you. Enjoy.

Cleave, to split apart, or to cling together. A butcher cleaves meat; a devoted couple cleaves to each other. Same word. Opposite jobs.

Sanction, to officially permit, or to officially punish. A government can sanction a project and sanction a country in the same sentence and mean two entirely different things.

Dust, to remove fine particles, or to add them. You dust a shelf (remove) and dust a cake with sugar (add). Nobody is confused. Everybody should be.

Bolt, to secure firmly in place, or to run away suddenly. You bolt the door, then bolt through it.

Oversight, careful supervision, or a careless failure to notice. The committee's oversight caused the oversight. That's a real, grammatical sentence.

Clip, to fasten together, or to cut apart. Clip the pages together, then clip the coupon out.

Buckle, to fasten securely, or to collapse. Buckle your seatbelt so you don't buckle under pressure.

Screen, to display, or to conceal. They screened the film behind a screen.

Weather, to endure, or to wear away. You weather a storm that weathers the rocks.

Left, remaining, or departed. "Who's left?" is genuinely two questions.

The unsettling lesson: a single word can absolutely betray you. Which, it turns out, is a whole game.