I almost forgot about this until I just now saw a similar thread on the MK64 board .
Well, this happened when I got my 9"74 lap record on VL2. I was NOT racing a CPU ghost. Anyway, the 5lap of that race was still fast enough for a replay (does anyone know how slow your time has to be before a replay is unavailable?) I chose to watch the replay, wondering if there was any room to push my new PR any lower. At first Bowser did everything in the replay that I did in the actual race, including breaking ice blocks-- but in the second lap, he started to behave very strangely, falling into the water and having to be picked up by Lakitu. He ran into the outer boundaries of the course, too. After an amount of time passed that was equal to the length of the race,
I was able to control Bowser in the replay!Apparently you're not truly watching a replay... the game automatically inputs the same commands that you inputted during the race, and possibly due to the nature of the ice blocks, the automatic commands may fall out of synch. I was disappointed because I couldn't watch my record lap again

I've heard about a glitch like this (I believe it was David Wonn who discovered the original glitch), although this is the first time I've seen it happen without a CPU ghost in the original race.
Has anyone else observed bizarre replay or ghost behavior like this, in SMK or any other Kart game?
Could this be the reason why MKSC won't let you save ghosts or replays on Retro tracks?