Wow, I'm shocked - in all our times at WDW, we've not only never been turned away with a past time FP (and we go at really busy times, like NYE), but we've never seen it happen, either.
Frankly, I'd be pissed if it DID happen. Look, if something is OK 99% of the time, even if they CAN enforce the rule, I think it's pretty arbitrary and rude to randomly enforce it. JMHO, but I'd put up a stink if the 75th time I used a FP after the time posted, some CM decides to randomly enforce the rule.
Again, just my opinion, but it's mine, and I'm sticking to it

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KC