In which case, their sergeant will tear them a new one, right after the custody sergeant has finished tearing their own hole because the careers of both of those people rely on supervising their coppers and supervising their arrests. If the custody sergeant has to release someone because the copper can't account for themselves, that is a very serious matter. The sergeant's can smell a bad arrest a mile away.
The copper has to stand up in a court of law, having sworn an oath, and testify on the reasonable suspicion or probable cause they had. If they are even suspected of lying, that's a gross misconduct in a public office investigation.
Assuming they weren't fired over that, any promotion hopes are gone, any possibility of involvement in major cases or crime squads, hope of a firearms ticket, advanced driving, or even overtime are gone. Their fellow officers will never trust them to make an arrest again.
It's not consequence free, I'm not saying it doesn't happen, or that some officers rely on you not knowing your rights, but it is a serious matter.