The following script will break a script's networking:
[UdonSynced] private MyUdonSharpBehaviour foo;
And causes the following error:
2022.10.20 02:49:28 Error - [Behaviour] Caught ArgumentNullException: Value cannot be null.
Parameter name: Received a null value when encoding a UdonBehaviour [...]
The error can only be found in VRChat's log file and was difficult to identify.