Umm yeah lemmie try again and I'll post console. The browser itself crashes, taking out all icons and the taskbar at the bottom until it restarts or I manually restart it.
Wrote 700:700 frames to videos/grangertest.avi
And now it worked fine this time, sort of. I had the old file & the new one in the folder, and it crashed, but the new one had a thumbnail showing a frame from the video, while the old one didn't. So I guess it was an error with the thumbnail...
I tried one more time to be sure, and I guess I was pressing a key to stop it, which caused the error. Thanks for the help.