This happened to me before, maybe there is one pixel somewhere with a gap in it that cant be seen? Still though surely there would have been more occurences if so.

It happened to Gatchan against Sami in a GP quarterfinal on Rainbow Road once.

My theory is that the Kart is going so fast that the game doesn't detect a frame in which the Kart is touching the jump plate. One frame, the Kart hasn't reached the jump plate yet, and the next, the Kart is already past it, effectively making it teleport past the jump plate. If the glitch only ever occurs when a Kart is under the effect of a Mushroom (especially with a heavyweight driver and 10 coins collected, though Aron had only 7 here), then this is probably the only feasible explanation, as there is no other scenario in which a Kart can go faster. I know there are similar glitches occuring in other games when a player is somehow able to travel at a speed not expected by the programmers; the Backwards Long Jump glitch in Super Mario 64 is probably one of the most well-known examples, where Mario can even pass through walls and star doors simply because he is moving so fast, the game never detects him touching them.

It really sucks that this glitch happens though, I can just imagine a CDM race being decided by a happenstance glitch like this (Gatchan must have really been pissed, hehe).
If this never happens at the MC2 crossing, it's either because (1) the zippers don't make your Kart go quite as fast as the shroom or (2) the jump plate at the MC2 crossing is a little different from a normal jump plate, either thicker than normal (so the Kart can't skip past it between frames) or it was just programmed differently (as BC bumpers are).

I must add that if my above reasoning is correct, then this glitch should happen more often on PAL, and might not happen at all on NTSC. This is because the Kart moves more from one frame to the next in PAL, making it more likely to travel the entire width of a bumper between frames and thus skip over it. In SMK, or any other game for that matter, due to the whole way that "frames" work, you are not ever moving forward steadily, but rather, you're skipping forward a certain distance each time the picture refreshes (30 or 60 times a second on NTSC, depending on the game; 25 or 50 on PAL). Thus, it's entirely possible that your Kart is not exactly in the same spot from one lap to the next. When you try to do, say, the GV1 platform jump, especially on PAL, you need your Kart to be close enough to make the jump on the last frame, because on the next frame it falls off the track. (Sami and possibly others have tested this on an emulator; sometimes there just isn't a frame where the Kart is close enough to jump from and this is part of the reason why it's so difficult especially to do multiple consecutive jumps on PAL without a boost, yet many people have 5/5'ed it on NTSC.) Similar logic applies when shrooming into a bumper. Normally there will be at least one frame where the Kart is touching the bumper, triggering it to jump, but if you happen to be moving as fast as a Kart can possibly move, and there's a frame where the Kart is a tiny bit short of the bumper, followed by one where the Kart is a tiny bit past it, you can certainly skip over such a small object especially on PAL when you're travelling farther from frame to frame.

I'm wondering now if it's possible to glitch through other small objects on the track, such as coins, when shrooming. Have you ever driven right through a coin but somehow still didn't collect it? Has anybody ever glitched through one of the "chocolate bumpers" on CI1/CI2?

It happens a bit more often on RR because there are two bumpers and there might be some kind of "blank" frame between them where you can fall. But on GV1 it makes absolutely no sense.

@Nick, why do you write "Kart" with a capital K when talking about the vehicle?

Just to illustrate the F-Zero X version of this bug I mentioned earlier, I found the perfect example:

http://youtu.be/8N7Ext1RpPs?t=1787

It happens during Wouter's AGDQ run of this year, at ~29m53s, exactly as he is explaining his fear for this glitch (tempting faith much?).