Libre, an ex-colleague had a simmilar experience to that while were contracted to do support work for BP.
He got called in the early hours and told a taxi was being sent to his house to bring him into Lndon urgently as the traders couldn't work. He rushed in and spent hours trying to work out why the code we were supporting had suddenly failed, all the time being berated by the a**hole of a manager and told how all of us were useless and this is why he'd argued against us being on-site and he was going to sue our company, etc. etc.
The investigation finally revealed that one of the in-house staff had manually deleted a folder from the server, somewhere out in the countryside. He'd checked it for a few nights and found it always empty. Rather than ask anyone, he'd deleted it. Unfortunately, it was used between 2:00AM and 2:30AM - to store the various downloaded share prices used by another application - after which time it was cleared ready for the next day. When the next process couldn't find this folder, it fell over.
Funnily enough, no apology was ever issued to the guy who got called out and it turned out (somehow) to be impossible to find out who had done the deletion, even though only one person was working in that department at that time each night.