(See the best response by Ter13.)
var/mob/M = usr
skilldescription(M,"Whirlwind Kick",jlevel,0,1,cooldown,"<font color=silver>Performs a quick spin kick, dealing [jdamage](<font color='#ff4500'>+[M.apow*0.4]<font color=silver>) damage to nearby enemies and slowing them for 1.5(<font color='#006400'>+[M.con*0.01]<font color=silver>) seconds.")

Problem description:
I keep getting the following runtime error:

runtime error: undefined proc or verb /client/MouseMove()

I'm not 100% certain the above code is what's causing it, but I don't know what else could be. I don't know why I keep getting this error, and it's hard to track down. I'd greatly appreciate some help with this issue. ^.^

What line of your code is that runtime pointing to?
Upgrade byond version to the latest, it's what caused the problem for me.

Recompile when you've done that.
It isn't pointing to any line, that's my problem. I'll try updating Byond
Most likely related to this

Would recommend reading through.
In response to SinfulPhoenix
Debug mode is your friend.
Best response
Go into Build->preferences, and check "Generate debugging information". When you are actively working on your game, you should always leave it in debug mode so you can... Well, debug.