I'm not recommending this, but I've shown up three hours late (long story) for an ADR and they seated me. So long as I have had an ADR, I have never been denied a table. That said, I'd obviously advise against showing up late, because you never know what will happen and, of course, it can really throws off the seaters.
The one other thing I'd look out for is the meal times and cutoffs. For example, a few weeks ago, I showed up to my 10:30am CRT ADR at 10:35am (I kid you not) and was told I'd have to come back for lunch service and pay the price difference, so be careful!