ModemManager used to open() and probe every tty device attached to the system. I had a 8-channel relay card with an arduino nano wired up with my desk to control the lights and disco ball, interfaced with a custom ascii-based serial protocol. connecting it to an ubuntu machine (where modemmanager was active in the default install) turned the 2nd or 3rd channel on.

This was generally infuriating, there are many arduino forum posts about modemmanager messing up DIY setups.

Upstream fix was changing modemmanager to work on a whitelist / opt-in approach instead of blacklist / out-opt. My fix was to switch to debian.