I've had a couple of experiences like this.

I went into the server room to find a layer of smoke covering the top 6 inches of the server room, but no obvious source.

It turns out that a server with 3 power units had had one burn out, unfortunately there was no way of telling which one of the three it was.

The engineer said that if he pulled the wrong unit out then there was no way that the server could keep going on one.

Well he pulled the wrong one out, and three cheers for HP the server did keep runnning!

He put the working pack in pretty smartish and replaced the dodgy unit.  Whilst he was there he decided to check the 3rd pack, just to be on the safe side.  He pulled the pack out and there was a sudden flash and a lot of smoke and a 2nd dead power-supply.

Somehow, through all of this the server kep running.

In case you are wondering, the server room was being expanded and therefore the smoke alarms were off.

In another situation, power was lost over the Christmas holiday.  One old SQL 6.5 machine had its disks seize once the temperature had dropped.  Note to the wise.  4 hour callout does you damn all good if your box is old and obsolete.