I also agree that it's possible to forget a LD (since it's possible to forget anything, including waking life stuff). In my experience, though, dreams with a decent level of lucidity are much less likely to be forgotten completely, especially if they're important (though low-level LDs and the initial non-lucid portions of DILDs can be a bit fuzzy for me at times). I've had nights with virtually no non-LD recall at all but several LDs that I remembered fine.

There are occasions where I wake up feeling like I just had a LD that I can't remember at all. In some cases, I have managed to remember the dream, but I usually found that I didn't really have a LD after all—it was a NLD about LDs, for instance, or merely a false memory in that dream of having had a LD, or something like that. So I just assume that if I were truly lucid, I would remember it.

Good recall still isn't a bad idea, though.