Good to see you posting wolftrax. That's an interesting theory about what could explain what we see. Without the original film, it is hard to be definitive about anything. I would disagree that this particular anomaly we're seeing in the film is an artifact though. I might have agreed if it was in just one frame, but this seems to "grow" from one frame to the next. (the size of the split/opening) However, I would submit that the bogus looking sideways knee joint, and diaper butt are probably more obvious signs of a suit. There are also other notable flaws IMHO but all of that stuff is subjective surely.
Excellent film clips demonstrating the infamous "flap". I've stayed away from that point for a long time because I thought it might have been an anomaly caused by foreground obstructions, like WP has suggested. But the one set of frames that has the box and triangle markers pretty much buries any hope of the white flash being a foreground obstruction. Unless the prior frame has been doctored to add in the part of the foot that would have been similarly blocked by the foreground debris.
It's so clear that the foot sweeps right past where this obstruction is supposed to be, without any hint of interference. The side of the foot is clearly visible, but when the heel sweeps by that exact location, there is that big white flash. Not only is there that white flash, it is bordered on the left with black. What is the white, and what is the black? It sure doesn't look like a case of motion blur because nothing before or after seems to blur like that, in either foot. Not only that, it is sharp edged, not particularly blurry or smudged.
Plus, there's also that seemingly excessive amount of bare heel and achilles skin showing in the frame prior. As much as I have avoided thinking that this was a split in the costume, it sure does look exactly like a split in the costume fur. It could be a spit for any number of reasons, whether the costume foot was wrapped with fur during construction, like a spat, with the seam in the back. Were it glued on, as was extremely common then as now, that glue could have separated and allowed the seam to open up.
I don't think it would be a split in the leg of the suit. If anything, the pant leg of the suit would probably help keep the split in place and keep it from getting worse.
I'll have to check out other segments to see if there is any indication of it starting to happen earlier. But quite honesty, it really does look like yhe fabirc separating from the costume foot. I sure don't recall Patty having that much skin showing up the rear of her ankle anywhere else.