Strangest one I was ever involved with went something like this:
Company had shut down for a week so we had all the HV testing done, After the testing and all the HV was back up and running my then boss was turning all the LV sub main breakers back on when one blew up when he turned it on!
Give him a nasty burn on his arm and the air turned blue.
So anyways contractor was called to test it and replace which they did, IR testing clear and breaker replaced turned on and it trips instantly (luckily no boom this time).
So they re-checked everything again and said it was ok and couldnt understand it.
So My gaffa concludes this is BS and wants a second opinion so sends us in to break the circuit down and test.
This perticular breaker fed a sub main board which had the breakers for 2 DB's and 2 Lifts which were all isolated so we knew the fault was between the sub station and the sub board.
So we stripped the sub board down to basics, IR tested the bus bars etc etc and found nothing!
I ring the gaffa and tell him its all clear and we found nothing, He shouts and swears at me that there has to be somthing (which I agree) But nether the less there is nothing.
By this time we are all getting a bit ****ed with this thing so in a bit of a temper I smack the side of the board and low and behold a 32mm metal hole saw cut out bounces onto the floor and you would never guess it was all black and crispy!
I have no idea how it didnt show up on the IR tests or where it was actually hidden, I can only assume the second time the circuit was energised it had blown off to a location undisclosed until I smacked the board one!
But after putting everything back together and turning the breakers back on everything was fine and dandy again