Actually segwit has cases like this. As I wrote in my last post, if you take a segwit transaction and strip the witness data off it, then it will be a valid transaction under the old rules but not valid in the new ones (because it is missing the witness). Adding restrictions like this (in the case of segwit, that the witness is verified in addition to the old-style signature) actually is the only point of a soft fork, and hence why you may call it "enforcing segwit". gmaxwell's mention of "safe" is very useful in the context of this discussion, but if you re-read his answer precisely, it only states that that means "extra care" is taken. My quote above, as it stands, is correct as far as I can tell.
ok, well it's difficult then to see how any soft fork can be implemented without an inherent risk of a hard fork. You're basically saying that pre-fork nodes are blind to the new rules, and so one can break the new rules without the pre-fork nodes being aware, that's perfectly sensible.
But you're still wrong. The old nodes are not "accepting the new rules", as you said. They cannot be accepting any new rules, because they are not designed to recognize rules that did not exist before that version of the consensus rules were written, Bitcoin does not (yet) have an algorithm that can see into the future!
What's happening is that they do not understand any new rules, and blocks with transactions adhering to any new rules are interpreted as "no signature required", i.e. anyone can pay. How else, logically, could old nodes be capable of accepting tx's that both observe and violate a new soft-fork? It's because they don't know what the new rules are that they are able to accept both, that's how anyone-can-pay can function as a backwards compatible upgrade mechnanism, because it enables nodes following only the older rules to ignore any and all assessment of adherence to new rules with 1 universal rule: "this transaction is valid, because it has no spending conditions". Saying that's the same thing as accepting new rules (which you did) is not accurate.
You seem to be dancing around the semantics, for reasons that are
1. Not obvious. What is this achieving
2. Not relevant to the OP anyway
I could instigate an attack on the ambiguities of the way you've worded things too, but I'm not going to do it. Why do you think that is?