It depends on who gets punished. If nobody but crooked cops can be proven to have covered up a crime, then only they are punished. But like murder and possessio

It depends on who gets punished. If nobody but crooked cops can be proven to have covered up a crime, then only they are punished. But like murder and possession of child pornography, knowledge of someone's criminal act legally complies the observer to turn that person in or they will be charged with conspiracy. You can cover up one or two crimes but not all of them. Eventually things get found out. You could help someone get away with something once, but if it's ever found out, you could potentially be charged with everything your buddy is up for, go to prison, and come out broke and jobless.