[gpm]

It's still evidence, just less credible evidence. Hopefully the prosecutor has other evidence as well.

It could also be the case that it's evidence that was collected by someone who didn't commit the crime, but would currently be excluded by the fruit of the poisoned tree doctrine.

[sanswork]

The only proof you have that it IS evidence is the word of someone who you know is willing to break the rules to secure a conviction.

[dagw]

Sure, and that is an argument you can make in court after the evidence has been submitted.

[hasenj]

I thought an evidence was an objective piece of data, not someone's word.

[bzbarsky]

An objective piece of data like what? A digital photo? A network traffic log? A printed photograph? A phone call recording?

These are _all_ based on someone's word, in a world of Photoshop, text editors, hex editors, scanners and printers, voice synthesis, etc.