I actually have a story about mentor mistakes, electrical polarity, and "
trust but verify".
My very first day as an FRC mentor, I was introduced to the head controls mentor and, looking for a job to give me, he handed me a package of addressable LED rope light he had just purchased from Adafruit, an Arduino board, a power supply, and said "here, take those two students and make this work".
I guided the students through a series of "what do you think we should do next?" exercises while learning the answers myself at the same time. OK, let's download the Arduino IDE software to a laptop. Let's Google for the spec sheets and sample wiring for these lights. Let's find some sample code. Let's get some jumper wires and a breadboard and wire something together. Let's double check everything before we turn it on. It looks good, let's turn it on...
Nothing. It just wasn't working. We checked it over again. Everything was wired just as it should be. Dead power supply maybe? Off to get a multimeter...
Lo and behold, the output voltage from the power supply was the reverse polarity from what was marked on the casing. The box said tip-positive but it was definitely sleeve-positive. A lot of "NO WAY!" and grumbling from myself and the head controls mentor.
So we reversed the power leads and AHA! Colourful flashing lights! Thankfully, the lights were reverse-polarity protected.
As I left that night, one of the parents came over and said "Hey, not bad for your first day!"