I've got some (These are in order)
1. When its just you and a hacker in a race (And they win of course)
2. When noobs with 8000+ VP come in (And they win of course)
3. People stick with a course you hate (Rainbow Road,
GBA Bowser Castle 3, Come on, where's the respect for Dry Dry Ruins?!?!?!?) (And they win of course)
4. When all of the Funkys and Rosalinas on the Flame Runners and Spears of the world race you and you are the odd man out (Where's the respect for Mario and the Dolphin Dasher?!?!?!?) (And of course they win)
5. Item ownage by hackers and noobs (And of course they pass you and finish the race in one freakin' second)
Worthy of mention: Losing 250+ VP in disconnections, Constant mirrored races, Listening to annoying characters (Miis, Donkey Kong and Rosalina mostly) behind you.