client.byond_version was not correctly handled as a read-only var.
BYOND Version:509
Operating System:Windows 8
Web Browser:Chrome 48.0.2564.48
Applies to:Dream Daemon
Status: Resolved (509.1319)

This issue has been resolved.
Running 509.1318

Code Snippet (if applicable) to Reproduce Problem:
vars["byond_version"] = 510
src << "byond_version altered once"

vars["byond_version"] = 510
src << "byond_version altered twice"

Descriptive Problem Summary:
The above code doesn't produce a "Cannot write to" run-time. Instead, it kills the client on the second alteration.

p.s. Are client.key and client.ckey meant to be changeable? Cause they are.

Excellent catch. Looks like there was a longstanding mistake in the code that didn't shunt this to the "cannot write client.var" result. I'll fix this in the next release. Please note this is a Dream Daemon bug, not Dream Maker, as it impacts server behavior and has nothing whatsoever to do with compilation or the editor; I've reclassified the bug accordingly.

The key and ckey vars are however meant to be writable.
My bad, I was thinking of 'DM Language', picked Dream Maker by mistake.

I took a look at the reference entry for client.key and ckey and in there it says they're read only. What would be the benefit of being able to change ckey and key anyways?
I should change the reference for those, then.

The benefit would mainly be if you used your own login system and wanted guests to be given names, or if you had a way of saying a certain user joining via telnet was really a particular person.
Lummox JR resolved issue with message:
client.byond_version was not correctly handled as a read-only var.