And I put it in beginners because it was a dumb mistake.
I was investigating a piece of production testing equipment that was called in as an IT issue because the software on the computer was failing everything that was being tested. During the various parts of the test, current snapshots are taken and compared against known good current ranges. The test operator has to activate certain functions in the device being tested based upon the prompts on the screen. (Turn this knob half way, and press this button etc) The software also has some ability to inject commands directly into the device's communication bus and during certain parts of the test, you will see the device being operated at a high speed with lights going off and on quickly as if a person was pressing buttons and flipping switches at 100 times their normal capability.
The current consumption for the vast majority of the test was not wavering at all, even when it should at times increase by 2000%
One of the suggestions by the support group for that device was make sure the power rails were good which is an absolutely reasonable thing to check. The previous day, one of the electrical maintenance guys said he tested the output voltages and they all seemed within spec. But I wasn't sure if it was tested under load or just free running.
My mistake? On the back of the unit were several twist in fuses. Each fuse was labeled as to which power rail it corresponded to. I put my left hand on the painted metal cabinet and bent down to remove the fuse that said "12A 1A supply" with my right hand and as soon as it was being withdrawn out of the socket I suddenly felt a rush of excitement, 60 times a second.
That's when I realized... Though it was for the 12V 1A supply, it was not fusing the output, it was fusing the input side. And confirmed when I saw 250V 5A stamped on the fuse.
So yeah. Even when you think you're dealing with small voltages on an unknown device.... make sure you're not the path of least resistance.