I'm pretty sure that "You can't have your cake and eat it, too" may be one of the most misunderstood sayings on the face of the planet. I frequently see people who are otherwise very intelligent misusing it, to the point that it's starting to become a pet peeve of mine. (Personally, I blame the public education system.)
Basically, what I frequently see people saying is, "Why shouldn't I eat the cake? Why else would I have cake? It is my cake!"
Granted, some of these people probably know very well what the saying means, and they are trying to be funny. However, it has started to get to the point where people are using this logic to try and make a serious point, and it really bothers me.
So, for those of you who may not realize, the saying does not say that you shouldn't eat the cake, or even that there's anything wrong with eating cake. The point of the saying is that once you eat the cake, it is eaten. If you eat the cake today, you can't expect to have cake to eat tomorrow. Trying to defeat this logic by saying "But why shouldn't I eat the cake?" is missing the point entirely.
The only thing this saying was ever supposed to mean is that you can't have two things that cancel each other out. For example, the phrase would be accurately applied to a person who was married and having an affair. You can't be monogamous, while still having lovers on the side.
It also is accurately applied to secretly gay ultra-republicans. But that's another post entirely.
Granted, people often use this saying when they feel like someone is getting too much of a good thing. If someone uses this saying on you in this way, a more accurate response would be, "I'm not. I'm just baking a new cake." This response also has the added benefit of being original enough to distract the ignorant person. I suggest running away while they are busy figuring it out.
Once again, NO ONE IS SAYING DON'T EAT THE CAKE. They are only saying that once you eat the cake, you can't decide not to eat the cake. But please, do eat the cake before it goes stale. No one likes stale cake.