Doesn't always happen, but in some cases, the shadow will imply that the shimmier won't succeed, but when used it works fine. This seems to be purely a bug with the shadows.
This happens at the height of the jump when the gap between the floor and ceiling is 26px. It also happens at most, but not all, gap sizes lower than 26px on the last frame before the jumper hits his head and becomes a faller. It doesn't seem to happen when the gap is larger than that.