Bad memory from a couple years ago. Debugging a machine under development at a customer site, which was luckily only a few blocks from the office, since I was there a lot, I would get random resets, hangs, and loss of debug information from the SWD probe. Swapping boards, debug probes, nothing we could think of fixed it and it was so random it was hard to track down.

Long story short, after it happened on one occasion when the project EE was also debugging something and he touched one of my probes, we found that there was conformal coat residue on some of the debug pins and the probes would occasionally vibrate onto an insulated section, causing loss of signal.

No idea why the board house put it on a prototype, but we probably lost a few dozen hours due to that one problem.