It's essentially when information that was used to identify the suspect is presented as confirmation of their guilt.
> The witness described a tall, white, black-haired man with a tattoo on his right ankle. Only .001% of the population match this description, so there is only a .001% chance that the suspect is innocent.
Of course, this ignores the fact that they compared the description against a database of one million people, among whom you would expect to find 10 coincidental matches.
For the same reason more data is required to select a model than to validate a model in machine learning.
> The witness described a tall, white, black-haired man with a tattoo on his right ankle. Only .001% of the population match this description, so there is only a .001% chance that the suspect is innocent.
Of course, this ignores the fact that they compared the description against a database of one million people, among whom you would expect to find 10 coincidental matches.
For the same reason more data is required to select a model than to validate a model in machine learning.