Ex post facto
| It's not just|
a good idea
Ex post facto is Latin for "after the fact", or, retroactively.
 In law
It most often comes up in the context of a prohibition on ex post facto laws or enforcement of them - that is, if the law governing an activity is passed after the activity occurs, one cannot be retroactively punished.
Ex post facto predictions (also known as vaticinium ex eventu, prophecy [derived from] the occurrence) are ones that are not made until the facts are available - i.e., explanations of events that sound "predictive" but are not made in advance of specific occurrences, but after them. Similarly, claiming given events fulfill prophecy after they occur allows wide leeway in interpretation, as opposed to interpreting the prophecy before the events and accurately predicting how they will occur.
It's easy to trick people into believing you made a true prediction, when in reality it was post facto. This can be done through clever misdirection and is often a trick used by mediums or other mind-readers. Cold readers can be quite good at getting people to reveal facts and then claiming that these facts were divined through psychic powers and were actually predicted. However, it doesn't always work. In 2009, mentalist Derren Brown attempted such a feat by "predicting" the results of the National Lottery draw. Using the same misdirection methods he uses in mental illusions and card tricks, he tried to convince the audience that he's predicted the results beforehand. However, most people seemed to see past the horseplay and clearly noticed that he revealed his "prediction" a few seconds after the result was shown - with the illusion being due to a camera trick.