Bought a cheap corner trowel from a flea market and it cost me a full day of rework.
The blade was so flimsy it left deep gouges in every inside corner on a big living room job. I had to go back, re-mud, and sand the whole thing, losing about $400 in time and materials. What's the cheapest tool you've ever bought that turned out to be a total waste?
Read a story once about a guy who bought a bargain bin drill bit set. The bits were so soft they just rounded off trying to go into a pine 2x4. He basically paid for a box of shiny metal crayons. That whole "buy nice or buy twice" thing hits hard with tools.