The probability of being a woman, given that you are pregnant is extremely high. But the probability of being pregnant, given that you are a woman is pretty low.
The two are obviously not the same. But consider the following:
The probability of getting a fingerprint match, given innocence.
The probability of innocence, given a fingerprint match.
Again, the probabilities are not the same but this example is much less obvious. Treating the first probability as equivalent to the second is a common (and sometimes wilful) error.
What is the name of this mistake?