Whenever I watch videos, my screen gets flashes of snow I'm using a 15 inch mid 2010 macbook with 10.8.4. It happens both on my cinema display and laptop display. It happens sporadically and had me questioning whether I was actually seeing it for a while. I notice mostly on youtube videos, but I think it's occurring from other sources as well. 

Somebody could be playing a prank on me (as my buddies are likely to do), but nobody has confessed and I can't really find anything about the issue.

Any Ideas?
Depending what kind of flashes you have it is unknown, but there is one we can take care off.

Go to your system preference and click on Accessibility.

Check if the Flash the screen on error is enabled.

If yes, disable it.