Bullets that kill. They resulted too expensive to fabricate, yet they still can kill you with a hit in any place of your body... like your arm for example where a normal bullet would cause a minor injury and you would survive.
An armored transport service. Legal paperwork killed this project.
Extreme gambling site for spanish bullfighting where people could gamble if the bull would kill or injury the bullfighter. Turned out that it was legal to gamble if the bullfighter would kill the bull, but not if the bull would kill him
Rent a security expert/hacker service. It worked too good till I got some call from people I don't like to work with, so I sold the company a couple of weeks after that.
Selling a complete phone system with phone centrals included to a government so the diplomatic phone system goes all encrypted. Failed because didn't bribe enough the correct person. Was too young then and still thinking that a couple of millions is enough bribe
Starting my own dating site. Don't know why, but I was never able to do it even that I'm pushing 900k uniques/month to AFF and making some nice money from that I'm unable to make my own dating site
And lots of others... the good thing is that I have lots of ideas and I'm never afraid to try them. Sometimes it works, sometimes it doesn't.