When the size of a dict changes while it is iterated over, we get a RuntimeError. But this is not the case on lists. For instance, the following code will loop until it exhausts memory:

seq = list(range(10))
for s in seq:
    seq.append(s)

Same situation with this code:

seq = list(range(10))
seq.extend(s for s in seq)

I would have expected such a corner case to be detected.