i've done this with two separate drugs...
i've od'd on heroin twice, once was because my dealer was giving me a lot of free dope and, being a damn junkie, i didn't want to waste it. i kept doing what he gave me until he said that he was going to drive me back to my house. i finished my shot and then blacked out and woke up to him frantically shaking me going 'you are NOT gonna die white girl!'. i didn't really know what was going on, but he kept shaking me and i eventually snapped out of it.
the other time, i don't know exactly what it was. i had been on kind of a dope binge for like, two nights--went through a hell of a lot. the third morning, my friend drove me home. i'd done one more shot before i left, but had missed it. the second i got home, i leaned over my porch and started trying to throw up...i managed to get myself inside and into my bedroom, and got into bed. i was just so drowsy and felt so strange, didn't really know what was going on, then i passed out (? i don't honestly know what i did). came to a few hours later in bed and felt like i had a really bad flu (i would have normally assumed i was dope sick, but this was after i'd quit doing it on a daily basis, and a two day binge would not make me that sick). i was having some really strange kind of hallucinations, i kept getting very disoriented, and honestly i was scared as fuck. i stayed in bed for the rest of the day and night and slowly came to sometime the next day.
my first overdose on ecstacy was last summer. i'd been out with some friends and someone had fed me two pills, before i even took them i was plastered drunk (i rarely mix my drinking and rolling...it never ends well). at six am, we headed home. i was kind of fuckered, but during our hour drive home i decided i wasn't fucked up enough and ate three more pills in rapid succession (with like, fifteen minute breaks in between--i was fucked up enough to have a terrible sense of time, which rarely happens to me). almost the second we got home i got out of the car and was like '....dude....what the fuck?' my boyfriend stood there, looked at me, grinned, and was like 'sucks when you take too much, huh?' he'd been watching me eat pills and decided to let me do it so i'd learn my lesson, since i'd really been pushing the boundaries all summer. i spent the next several hours curled up in my friend's bed talking some strange nonsense...it was kind of like my brain was watching my body from behind a pane of glass, i could hear myself talking and could see and everything, but i didn't feel like i was the one in control. i don't know how long it took for me to come down at all, but eventually i stumbled into the bathroom and threw up. it was disgusting, vodka and cranberry juice and pill vomit.
...since then i've never been able to swallow a pill without vomiting (don't know if it's due to that, but it seems likely).