To reproduce: Play attached level. Assign diggers such that they take away the full 9 pixels at the edge. Notice the 3rd Slider from the left does not slide at all. (The other three display varying-length finite variations of the
Slider-Digger infinite loop.)
On the surface, this seems awkward. However, it actually makes sense to some extent if viewed in the context of "what would happen if a more significant amount of terrain were removed, instead of just 1 pixel" - it's just that doing this with a digger is a really, really precise setup compared to, say, a bomber. Although even in these other cases, the destruction would have to occur exactly as the lemming walks off the edge - but there's no logical other behavior if, say, a bomber takes out a huge chunk at that exact time; the digger is a weird case where you
could argue there's a logical behavior.
My preference overall is to leave this as-is, but I feel it does need to be put up for discussion at least. An alternative could be to have a 1px tolerance, but if more than 1px of terrain has been removed, this behavior continues - however, that's an edge case that's pretty much specific to the digger (and a few pixel-precise setups with other skills), so I'm not sure how justifiable it is. This is why I prefer the "do nothing" approach personally.