If I spent a little time searching I could probably find an example of a less effective robbery, but this'll do for today:

The suspect said, "Give me your wallet or my buddy will beat you up," but the victim didn't see anyone else with him, according to the report. 

Following the threat, the suspect reportedly made a gun with his thumb and forefinger. The suspect had his other hand in his pocket, but the victim believed he was just using that hand to keep his pants up.

According to the report, the victim walked on by the suspect, who appeared to be drunk or high, without giving him any money.

Strike one: invisible thug. Strike two: non-dangerous finger. Strike three: hasn't even managed to steal enough money to buy a belt. Yep, safe to just keep walking. Probably okay to roll your eyes, also.

This frankly seems too weak to have justified a police report, but I'm glad the "victim" filed one anyway.

Okay, I probably should have said "less competent robber," because arguably all robberies that fail are equally (in)effective. Here are two examples that would be on my bottom ten list, but at least these guys put a little effort into it.