Working for a submarine telecoms cable manufacturer in university holidays when I was required to get a bit of "shop floor" experience. The company was pretty good about letting me have a week in several different departments which ended up in the "final testing" area of the shore-end equipment - think lots of full height 19" telecoms racks.
I was doing the final tests on the "spare set" for something like TAT-9 (which dates this whole story if you look that up) prior to delivery to BT. Along with various messing around with optical fibres and high speed data testing equipment, etc. the final tests were "how much current does each shelf in the rack draw?". The technique was to grab one of these wheeled stair/ladder things, go up to the top with your multi-meter to where the fuses all were, set it to current mode, put the probes into the terminals on each side of the fuse, hold them with one hand whilst you unscrew the fuse with the other. You can read the current, then screw the fuse back in and write the current down on the test sheet - all of which means you get the normal operating current without anything being turned on/off. You then rinse-and-repeat down the line of fuses at the top of the rack.
The final measurement was supply voltage to the rack. This came from a standard telecoms 48V DC supply (think big pile of lead-acid batteries).
Notably the instructions in the test did _not _ include the helpful advice to set the multi-meter to voltage mode first.
As a multi-meter in current mode is a basically just a thickish piece of wire the resultant spark and clunk from the circuit breaker on the battery pack were quite impressive. Followed by the extensive "rattle" as every relay in the various PSUs clicked off and the descending whines of the many, many fans in _all_ the racks that were currently under test ...
Apparently I went white-as-a-sheet which I suspect did something to dampen the laughter.
Thankfully the foreman scuttled over pretty quickly to put me out of my misery and said something like "Don't worry - it's just the circuit breaker, the racks will be fine. Everyone does this - once. You forgot to set the multi-meter to voltage didn't you?"