Graphics glitches always show up on machines that use nVidia graphics cards.
I can't determine if certain generations of nVidia cards have this glitch, or if all of them do. I don't think the OS is the issue.
On my own machine, Windows 7 and an AMD HD6870, I am getting no graphical issues.
However, on other machines that I believe have nVidia graphics cards, severe glitches with the lighting system occur.
My lighting system is pretty complex so it's difficult to track down specifically which parts of the code would cause this.
There are actually several different glitches that happen, but happen independently from the others. It's difficult for me to track these down because I cannot reproduce this on my machine.
Numbered Steps to Reproduce Problem:
Run the game and walk around: http://puu.sh/qj3JE/e71deead7c.zip
Actual Results:
The following three screenshots were taken on the same machine.
This is what I've dubbed the ghosting glitch. I can't reproduce this

Here, I can't tell what had happened to the lighting; this is a second glitch

However, when this playtester moved a little towards the east, this second glitch disappears.

The playtester reported that the second glitch sometimes come and go almost randomly based upon the character's position in the map.
Here is a screenshot from a different tester's machine that also exhibits the ghosting glitch.

Here are some short videos recorded by a third playtester, also running an nVidia card. These depict both types of glitches.
Video 1
Video 2
Expected Results:
The following screenshots were taken from my machine.
These screenshots are taken to attempt to show what the different positions above should have looked like.
This screenshot was taken to reflect the first screenshot from above.

This screenshot was taken to reflect the second from above.

None of the reported glitches occur on my machine.
Does the problem occur:
Every time? Or how often?
Every time on any afflicted machine.
In other games?
Unknown
In other user accounts?
Yes
On other computers?
Yes but not my own
When does the problem NOT occur?
When you run the game on a system without an nvidia card...?
Did the problem NOT occur in any earlier versions? If so, what was the last version that worked? (Visit http://www.byond.com/download/build to download old versions for testing.)
Don't know. Some playtesters have reported this on versions as far back as version 510.1345. However, I have confirmed that some of the previous versions used to yield no glitches. This will take a long time to track down since it was many months ago.
Workarounds:
Don't know any.