Smoke grenade.
Of all pyrotechnic stuff one my friend showed most interest in smoke grenades.
When it was his birthday, I told this to 2 other friends when we were discussing a present and we've decided that it was a good idea. I mean, what could possibly go wrong?
We didn't want to make something weak and silly so we've chosen size of 0.5 l (16.9 in freedom units) beverage can, which would be almost fully filled with active compound, save little space for ignition. We had to visit several pharmacies to get enough ingredients.
We bought ingredients and materials for casing (also some other stuff so it will not be only present), gathered at one of those friends' apartment and started our crafty endeavour.
His father had some guests so we occupied vacant bedroom.
I was making the casing and ignition while guys were grinding up active ingredients. I joined them later, because grinding that stuff is takes a lot of time and we needed lots of it. When we were done with that stuff, we assembled the thing, vacuumed spilled stuff and just let the device be there while we proceeded to make other stuff.
Little did we know that reaction starts immediately as you mix ingredients. I just goes really slowly so you don't see it. But it goes. And it produces heat. And as ingredients heat up, reaction goes faster and there's more heat and reaction goes even faster. So basically it just lays dormant for some time (longer if you put it in the freezer) then goes WHOOSH really quick. It also goes off immediately when you heat it up or try to ignite it.
It was pretty warm at friend's house.
As we were making other stuff, smoke bomb went off.
It startted making hissing sound which was accompanied by flames jetting out of casing's vents a second later.
Luckily we haven't wasted more time than it takes to see this thing going off and say something expressive under your breath. One of my accomplices bolted to close bedroom's door while I grabbed the thing, stepped to balcony and threw flame- and smoke-breathing device out of the window.
The room was not completely filled with smoke, you could see through the room on the floor level and near walls, but waist level and above in room's center were impervious to sight. You could not see ceiling lamp but you could see its light, blurred and smudged into little glowing cloud where ceiling lamp should be.
And the smoke was toxic.
Well, it was no ordinary smoke, it was something chemical and it was nastier than usual smoke, but I'm not sure if it really was toxic but anyway.
We got lucky that vacuum cleaner was in our reach. I placed it on balcony (so its exhaust would be in ventilated space) and I started my tributeto Ghostbusters, capturing the cloud of smoke with vacuum cleaner. It worked! At around the same time, outside, device exhausted all the compound and stopped.
Either vacuum cleaner's filters caught the smoke or it was vented through the balcony but it was removed from the room so it was safe and we could see the aftermath.
Aftermath was better than it could be: this time we didn't burn anything and the only damage done was brown stuff that was generated in the reaction and sprayed by fire jetting out of the device. That brown stuff is hot so it leaves burns and also chemically active so it leaves chemical burns. Luckily none of us was hit by it, there was some splatter on the floor (easy to remove) and some on a little carpet (basically ruined).
Well, we celebrated our insight that prevented us from relying on one gift and make more and went to the party.
Honestly, we got lucky: party was at birthday boy's GF's apartment and it was rather cold outside. Had the device lingered for some more time, we might be able to successfully bring and present it - just for it to go off at the party. I suppose that would not be a good present after all.
TL;DR: homemade smoke grenade accidentally went off and it was awesome.