Joe Devlin knew the alcohol on the bottle removed his fingerprints. He thought he had removed every piece of paper in the hotel suite. He left the bottle with its slogan, something like "Give every man his full" as both a joke and a warning to anyone who would betray his precious cause. He never realized anyone would notice the scratch or make anything of it. He may not have known that no 2 diamonds are alike, therefore the scratch made by one can be traced back to the diamond. And he never thought the cops would finger him in the arms dealers death.

Joe (Thomas, not Devlin)

I can only quote Sgt. Hoffman from
the episode "Negative Reaction" who
tells Columbo:
"Everybody makes mistakes,
that's why the jails are all full up!"