Smarter clients (and I agree that there are plenty of not-so-smart clients), will actually double-check on these things before buying in various ways. They can, for example, request an evaluation version to use, check with the support department exactly what's actually supported, or simply have them demonstrate exactly how the product can meet that requirement.
I'm sure there are more ways of doing that too, though, so these are just my observations. And yeah, don't trust the sales guys :)
Practically though, if you've worked in sales, you'll know that what the OP is talking about isn't just something that happens often, but is basically the life-blood of some rather large companies.
Now, I dislike it because it puts the impetus on the developers to deal with it if it goes wrong, and that sort of crunch-time stress is just crazy, but it's also not something that the OP has come up with on his own...