Once a guy was constantly hitting me with blue shells or invincibility (don't remember which). When I say constantly I do mean literally constantly, I couldn't even start moving before the next one came. Then after a while everyone got disconnected from the game I think.
So yes, there are some weird cheaters out there. Fortunately it only happened once to me.
My Mario Kart Wii friend code: 2707-1866-0957