I've certainly had it happen but there really is no solution that is entirely acceptable. The "correct" (as in, you do not operate in moral and/or legal gray area) answer is almost certainly "buy it again" but in many cases it really isn't an option and if it is, often the copy one might purchase is used anyhow. The less "correct" solution (in that you operate in a moral gray area) is to simply download an image of that disc. For most, such a move is entirely moral as they have already paid for said game.
I have personally used the latter more than once. I wanted to finish Planescape: Torment for example but was missing the second disc. Thus I simply downloaded an image of said disc to bridge the gap. Of course, I have also re-purchased a game because of this as well, hence why I currently own three copies of Diablo 2. Or rather, I have one complete copy and several copies of specific bits of it.