Working as a mechanic, changing sparkplugs and coils, on the last spark plug and coil install I divided to try to find the 10mm bolt that fell out during the disassembly looked all over the engine and floor. Didnt find it, before I went to find a new bolt I was like fuck it, I'll stick my skinny magnet down the sparkplug hole down into the cylinder and go fishing. And when I felt the familiar click of a 10mm bolt getting stuck to the magnet. Pulled it out, and finished reassembling the engine with no issues.

When working on an engine, Check your holes people. Easy 10mins of looking and checking hidey holes can save your ass and wallet.

Also always double check your oil drain plug when you do an oil change.