Spending a day figuring out why a new routine wasn't called only to realize there is another copy in /usr/local/bin of the same program that took precedence in the path.

Having the cleaning lady unplug the air conditioner to vacuum inside a glass enclosed server cage (fancy office) and to walk in there in the morning. It was like walking in to a wall of heat, amazingly most of the machines still worked, but we did end up replacing all of them.