Building just what you need, even when you know it's wrong

I just completed some functionality that I know is incomplete. Sure, all the tests pass, but I know there are other tests that could be written. But since we don't have time to make the tests pass, I'm just not writing them.

The functionality basically merges values into a document. Think "mail merge" type functionality or smarty-style tags. The current implementation will work for all of the "official" user stories we have documented. However, I can think of certain situations where, especially if escaping is involved, the merged output will not be what is intended.

So, should the time be spent now to make sure that any possible input can be handled? Or do we stick with something that may handle all of our needs, and add tests only when we run into situations where a customer need additional functionality?

Regardless of the "right" answer, we're moving on . . .