Technology has always been about making the world a better place: one with less of your enemies in it. Jet engines, computers and radar were invented by geniuses in wartime, but we're looking at the inventions of idiots during peacetime, and the result is more ridiculously contrived self-defense systems than Jackie Chan's career. Some were so stupid they weren't even given patents, and the US patent office has given patents to a stick and a vaginal bong. Unfortunately, neither was a euphemism.
Inventions don't grow on... huh.
The Self-Defense Memo PadPatent Number: 5,823,572
Even the disembodied mugger's head seems confused by this invention. The inventor appears to believe that the only problem with using a notepad as a weapon is that it's too hard to hold, which should give us some idea of his motor skills. He also thinks wrapping your hand around a thin, hard rectangle before punching someone is a good idea, proving that he understands less about violence than unborn children, who at least know how to kick people in the gut.
Violence and memos really don't go together.
While the confusing series of numbered features wants us to believe that the mugger's eye is somehow part of the invention, the only difference between this and a handful of Post-Its is the carved handholds in the side of the pages. Meaning it's not only useless as a weapon, it's actually counter productive as a notepad, assuming you don't want everyone you leave a note for to know they can just beat you up. The patent also suggests that the memos are useful for jotting down a description of your attacker, which is so likely, we're surprised they don't suggest that you also sketch the criminal's getaway unicorn.
When It Would Work:
If you can convince the mugger to pose like the one in the drawing, including not having a body, you might be OK. But if your enemy gives you unfettered access to their face and you still need extra weapons to win, you're probably screwed. So really, the only way this pad could defend you from attack is if you wrote down your PIN number and handed it over with your wallet.
Revolver Flick-BayonetPatent Number: 946,132
In the most tragic firearm-based mistake outside of thinking your gun's unloaded while you clean it with your penis, the cartoonishly-named Henry H. Hull of Ohio designed a revolver with a switchblade. Because after you've pulled a gun on someone, that gun then pulling a knife will be what really scares them. If you've never fired a handgun, attaching extra weight to the end of the barrel is worse for your aim than drinking a bottle of tequila.
Worse, it's not an attachment for existing revolvers. The switchblade is built into a protrusion from the barrel so you have to buy an entire new gun just for this idiotic addition. A gun's entire deal is propelling extremely unfriendly things at your enemies, but it's meant to be bullets by explosions, not a knife by a little spring. It's a worse weapon upgrade than sprinkling sneezing powder on a landmine - there's no conceivable enemy it will be effective against, and there's better than good chance it'll get you killed.
When It Would Work:
Obviously, the idea here is to have a little left over fight in you once you've emptied your ammunition. But if you're at the range where you could use this knife and still missed all six shots, you're honor-bound to just let them kill you.