Giving people a fright is acceptable, putting them in danger is not. As Rod says, it's a pretty narrow groove. Shrapnel from an exploding cap just might get in someone's eye, but it's pretty unlikely.
Someone some time ago talked about someone putting a short on a large electric motor, now that is plain stupid.
I once found some full-size Edison-base flashbulbs at a garage sale. They looked just like incandescent bulbs, other than being full of (I think) oxygen and magnesium wool. I had fun screwing them in in place of ordinary light bulbs. When someone flicked the light on - PAFF! Blinding flash!
I had a long long discussion working that out.
Fun is when you are on the edge, when you think you are in control but everyone around you think you are not. If things are too easy, that is boring. Too hard, that is frightening. Fun is just that narrow groove.
One of my classmates seemed to be fond of looking in my desk. So I left a well charged electrolitic for him. He picked it up, screaming and shaking his arm as the charge bled down. When it finally allowed him to release his fingers, the cap hit the window with enough velocity to punch a neat little round hole in it.
We had to split the window cost, me for leaving a trap, and him for being stupid.
He still got in my desk. Even after the exlax cookies.
some people are slow learners.
While in school we used to place a capacitor across the 600V three phase lines and wait for the teacher to turn on the power. It always got a good laugh when the cap blew. I'm sure the teacher knew what was about to happen as it would have appeared strange to see the entire class wearing safety goggles. I'm most postive he knew but acted clueless anyway. Those were the days.
The Other Tesla David Blaza5 comments I find myself going to Kickstarter and Indiegogo on a regular basis these days because they have become real innovation marketplaces. As far as I'm concerned, this is where a lot of cool ...