I ran into some weird problems with some circuits I've been connecting. They wouldn't work when they obviously should have. To make a long story short, the jumper wires that came with the starter kit have ... believe it or not ... some kind of plastic coating covering the part that's supposed to conduct! Apparently sometimes it scrapes off when inserting it into the breadboard, and sometimes it doesn't. Good grief.
I attached a picture of some of them held under my magnifying glass light. You can see the plastic on the wires, and where I tried to scrape some of it off with my fingernail.
What's up with that Adafruit?