I have a setup where I am sending camera data over UDP that was working yesterday. Today I am seeing strange behavior:
1. My first attempt to compile threw a bunch of weird errors. I closed the IDE and retried and everything compiled again. However, compile times are really long for some reason.
2. I then began seeing all the UDP data arriving scrambled and serial data quits right before I initialize ethernet. The serial hang seems to suggest the Teensy is hung, but I keep seeing UDP traffic of the correct format but filled with garbage.
3. I power cycled my PC. The problem persisted. The Teensy would not reliably restart. It seemed that a phantom COM port had appeared. I deleted it and re-attached the Teensy. Behavior then returned to normal.

So...it seems like the Teensy may have been using two COM ports: It starts out using one, and then switches once the ethernet is enabled..and this may have been caused by some one-time library messup. It's only ever been in USB serial (single) mode, so I'm not sure how this is possible.

This chewed up a lot of time. If I see something similar in the future, what debug steps can I take to shorten the time to resolution?