Umm..there's a lot of things.
1. Getting hit with more than one item at a time on wifi.
2. Getting hit with a red shell/lightning when i'm about to use the dk summit/dk mountain shortcut.
3. Getting bumped off the road by someone else on wifi.
4. Getting hit with a red shell even when i have something behind me.
5. Getting in 1st place and then a blue shell comes.
6. Hackers of course.
7. Getting ran into while doing a wheelie..slows me down alot.
8. Start using a good item and lighting strikes.