Well, you may not blow up the world, but I've had the fire department show up.

I was trying to find a new untagged POTS line installed on an old 66 block. This was a very old industrial building with 9 sprinkler risers. Come to find out they had these risers monitored by an old McCulloch loop (direct wire) to the fire department. When I clipped my buttset on that pair, it was enough of a short to cause an alarm.

Problem was, I didn't know what had happened until I had firemen standing behind me in the plant. They told me that an alarm came in on an old monitoring board that they weren't even sure that still worked or not.

Needless to say, with them there, I refound that pair and tagged it.