I was installing a new bathroom in a cupboard conversion and every day the customer complained about a foul drainage smell. It happened at the same time daily and it really was rank. If you imagine several thousand cows farting at the same time, it was that bad.
I cut into pipes, sent cameras down pipes, drain rodded sections, all to no avail. The smell would linger for a while, then disappear, only to return at the same time the next day.
Then one morning in sheer desperation in a basement corner of the building I forced a lock on a door, that had always appeared locked to me and the customer didn't have a key for.
When I opened the door, I was nearly comatised by the smell, but the cause soon became apparent.
Sitting bold as brass on the loo with his pants round his bloody ankles, and sh*tting for Scotland was the lad I'd hired to help me on this job.
He didn't last long.