"While it is very rare, it is possible to forget about a lucid dream."
You can have a lucid dream without realizing it, or even dream with some degree of lucidity, without it being a "true" lucid dream. The attraction of having lucid dreams is that, although you are in a dream, it feels almost exactly like reality and you have to use "reality checks" to make sure that you are in fact dreaming. In order to get to that point, you need to be able to recall your dreams very well, so it is difficult to unintentionally dream like that, or so my understanding is.