The most self-incriminating thing a suspect can say to a law enforcement official.
A suspect who has a substantial amount of evidence linking him/her to a crime will act completely oblivious to the matter.
Saying such a phrase will destroy a suspect's credibility since he/she is being non-cooperative and is obviously lying.
Officer: We've got your DNA on the dead body, your fingerprints are all over the murder weapon, and we have three credible eye-witnesses saying they saw you pull the trigger!
Say this (question “WHAT”) pause (then say “no way, or no fuckin way”) as a response to what you just saw and or heard. Usually after something amazing, unique, or even a correction to make life easier etc.