Strange 'Assert' popup box

This just happened to me for the first time in two years I’ve had GMod about two minutes ago:

http://img442.imageshack.us/img442/7105/wtgft.jpg

Any ideas what it means?

I clicked on Break In Debugger and it let the loading sequence for the server work for a few second, but after I got in the map it crashed and didn’t pop up any error boxes. I terminated HL2.exe in the Task Manager after that.

**Specs: **
Intel Pentium 4 3ghz
2gb RAM
ATI HD 4650 1GB AGP

(Will provide more if needed)

Did you get this popup again after you joined a server?
I’ve never seen this popup before for me…

I created a Singleplayer game and it popped up.

I tried it again after I did the OP and nothing happened.

Strange. :confused: