There's word that cheaters (you bastards!) are apparently moving in full force on Mario Kart Wii. Either that, or there's a bug. The Time Trials mode is getting hit hardest with all sorts of impossible times appearing. According to Mercurys Phoenix at the Official Nintendo Magazine forums writes:
I've only seen it on the Snowboard track - a guy with a time of 53 point something or other seconds. There are about three other people with exactly the same time. Like others have said, on the replay the guy just does one lap (excrutiatingly slowly) and then turns the opposite way just after the finish line. Then the replay doesn't stop.
Personally, I think its a bug. I've only seen it on one track and there are others with the same time.