Any time you try to second guess someone and the person is aware of it. You may reason that, based on previous experience the person will do X. The person may deliberately decide to do Y just to throw you off. You may anticipate this and plan on Y, but if they expect this of you, they may switch back to doing X. And so on, and there may be more possibilities other than X and Y.

When you choose not to decide.

When there is insufficient information to make a reasoned choice you might decide to toss a coin instead.