I have found that erasing a mistake in a synthesis or retro-synthetic analysis does just that; it erases the thought process which led to the mistake. I purchased a whiteboard and began to work out reactions there, and rather than erase my mistakes I’ve taken to just marking them out in a different color, say red, then rewriting the problem backward and forward the correct way once finished on paper. This way I can still see what I was doing wrong the first time in order to get to the root of the problem and avoid the same mistake in the future. I go back over the material/notes to determine the reason for my initial misunderstanding of the mechanism which leads me to a deeper understanding of it. In turn, I am better able to apply the concepts to any related reactions. And of course the concept is only half of the battle. I still make flash cards with starting material and product on the front and reagents are listed on the back, and either starting material or product with reagents and the mechanism and result of either synthesis or retrosynthetic analysis on the back. But the whiteboard has been my best friend, and has saved a lot of trees.