I played HOMM3 for 5 hours straight when I first encountered it. I know that sounds like nothing, but unbeknownst to me, installing it had reset my graphics drivers to set my monitor's refresh to 50Hz.

50Hz is one of those things you can see if you look at it in your peripheral vision, but not so much straight on.

After 5 hours I was screaming in pain my eyes hurt so bad. I couldn't figure out why they hurt and I couldn't stop playing either.