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 :)