Don’t think of a purple elephant you know the game. It is practically impossible to manage this seemingly simple task. We will think about the suppressed thought, probably every minute or so. Why?
In fact, the more seriously we put effort into avoiding the thought, the more it comes to consciousness. In order to avoid the thought, we have to keep monitoring whether the suppression is working. We, in effect, keep asking ourselves whether a purple elephant is anywhere near our consciousness. Every once in a while, that unconscious monitoring pops the thought into consciousness.
This sort of mechanism can explain some social gaffs and freudian slips. We are just trying too hard to avoid certain subjects/words. If we were less nervous about those subjects it would be easier to avoid blurting out the unacceptable remark we are determined to avoid. I think some people call this ‘don’t mention the war’ effect after a famous Fawlty Towers episode.
I heard a story of someone trying to learn to ride a bicycle. They were finally staying upright and feeling pretty good. After a few moments there was a post along the path and they concentrated on avoiding it. The more they told themselves to avoid the post, the more they were aimed at it and in the end hit it (dead center).
What comes into the center of consciousness is what is important either because it was not predicted (a surprise) or because it is part of the on-going task we are trying to accomplish. Usually these are referred to as bottom-up and top-down steering of attention. We have to be careful not to turn a no-no thought into a top-down focus of attention.