I was working at an assembly plant for plane motors (the big kind) and one of them literally blew up in the test bed. There was chunks literally embedded in the safety glass, it was a huge mess.

Turns out someone left an orange rubber mallet inside of it. Over the course of a year, they reassembled the shredded mallet and traced it back to the toolbox that used it. The guy lost it and instead of reporting it and disassembling his last job, he just stole one from an other toolbox.

Not mine but my buddy used to build kayaks. One of the employees took a dump in one of the kayaks and it only got caught because of a random QC test. I always giggle thinking of the client who would have received it.