My client was missing a mod, and the following two errors were spammed in the server's console:

http://pastebin.com/gAd1nNkS

Thought I'd let you know this is happening; it doesn't cause the server or client to crash, but it spams the console a bit.

It's not the mod reject, it's that the client disconnects before actually being connected

Ah derp, I was focused on some other mods at the time that were throwing stacktraces, which made me just automatically assume this was an error due to ElecCore.