One of the big ones is people just over clocking way past what’s sensible. Another is stuff like https://www.reddit.com/r/pathofexile/comments/z3xf5r/msi_aft... or https://www.reddit.com/r/pcmasterrace/comments/ob5jam/msi_af...

Other overlay apps like discord https://www.reddit.com/r/discordapp/comments/1jncrlc/new_ove... have similar issues - the way they work is by drive hooks and drawing over the application; depending on how close you are to the limit and how the game loop is coded this can very easily (and often does) cause micro stuttering, and in cases like discord causes massive performance problems regularly.