ITT: Morons claiming a file being named "server.cs" gives a magical protection barrier against it breaking client code.
If you run a server.cs file on a local server (AKA: non-dedicated) it can absolutely break stuff. If you don't believe me, make a server.cs file and run its add-on:
function servercmdProof(%cl)
{ commandToServer('messageSent', "As you can see, this server-sided file is running client code."); }
Fact of the matter is, Fillcan is the cause. I have done tests on this myself and was able to reproduce the error when on a single-player server with only default add-ons and the Fillcan enabled. Disabling the Fillcan fixed the problem.