I threw out a lot of old Mac products because I kept forgetting about B2M. I also regret not thinking more about which products were discontinued before I got rid of them. I also wish I'd figured out earlier that it was possible to wear more than one colour of eye make-up at a time. I can't think of (or at least I don't want to think of) the number of times I went out looking strange because it looked like I had half my make-up on.
But my biggest mistake was purchasing a product at Sephora that supposedly allowed you to use regular eye shadows as liners. This sounded great to me, so I went home and started using it right away... and ended up killing a lot of my dark eye shadows (including some LE ones). IN this case, the girl at the store actually told me that it wouldn't hurt my shadows, but that turned out to be a load. It didn't even end up giving very good colour payoff, on top of all that. Now if something sounds too good to be true, I know to run like hell.