"Be careful not to overdo the "false readings" for Detect Evil."
Yeah, their contact was a lawful evil necromancer... totally valid reading, but since he hadn't broken any laws (in public) and the fact that the paladin used smite to knock off nearly half the guys' hp in the first strike and...
I remember a session of 3.5 a few years ago when the party I played in had an overly zealous paladin. When meeting a contact with some information, he auto-detected, determined the contact was evil and smote him before saying anything to the npc (except "Die evil doer!").
One murder trial...