I've had many, but I'll just list me worst four.  

1):  High School Biology lab.  Dissecting a giant clam which has been preserved in a 30% formaldehyde solution for quite some time.  While standing by the mouth of the clam, my lab partner accidentally pressed down on the stomach of the clam.  A jet of formaldehyde squirts out of the clam onto the right side of my face.  Thank god I was wearing goggles, because they were covered in the formaldehyde as was my face.  The skin quickly turned 'dead' and started falling off my face in sheets.  I was raw for quite a few days after that.  Looked like a really nasty sunburn, but didn't really hurt.

2):  High school chemistry lab prep.  Making a solution of sodium silicate outside of the fume hood.  (STUPID!) and while weighing out the sample the powder kind of 'poofed' into the air right into my face.  I was coughing for quite a long time and silicates aren't good things to have in your lungs.

3):  High school chemistry lab.  Bending some glass tubing which had been used years before for bromine collection.  While heating the tubing, bromine leached out of the hot glass and vaporized into the air.  I was standing in the path going from the forming bromine to the air vent overhead.  Smelled REALLY bad and burned my sinuses.

4):  Worst one ever was college analytical lab.  I really wasn't thinking that day and I paid dearly for it.  Doing a lab involving the analysis of the nickel content of a US Nickel.  At one point in the experiment I accidentally opened up the bottle of concentrated ammonia solution outside of the fume hood.  Eyes teared up for a while and my nose wasn't too happy.  At the end of lab, when I was in a rush to get out of there, I forgot to label a beaker of concentrated H2SO4 that I had.  So when it was time to clean up, I just put my thumb into the solution thinking that it was only water.  It wasn't.    My thumbnail immediately started to fizz a bit and the burning sensation started in.  I rush over to the sink/waste bottle, still holding the beaker of acid in my hand, and in my hast put my forearm over an open bottle of concentrated nitric acid.  There was a puddle of the acid on the top of the bottle opening.  It found my forearm.  So I quickly put down the beaker and wash off the acid on my thumb.  It actually hurt more when the water made contact and I began washing it off.  In the meantime, the nitric acid was 'acting upon' my forearm and left me with a nice scar there which turned yellow-brown for a good long while.  My thumb recovered quite nicely and there are no noticeable scars on it.  I guess that's because my skin was so dry that the H2SO4 really couldn't eat the layer of dead cells.  The HNO3, meanwhile, was a bit more vicious and I have a nice little scar there.  

I've been pretty lucky in lab and haven't had too many accidents.  A couple of small fires from various things.  The most traumatic was impaling my hand with a syringe about a year ago.  It went in there pretty deep, but didn't really hurt as much as it just surprised me.  I quickly removed it, but I had to inject the stuff into a reaction or I would waste the reaction I had just spent an hour setting up, so I had to add the stuff before tending to my hand.  By the time I got a look and my hand the glove I was wearing was pretty much a balloon of blood.  Good times indeed.  The muscle in my hand really tensed up and was throbbing like crazy.  The puncture wasn't all that bad, but I lost a lot of fine motor control in my left hand for the next week or so.  I don't have a scar or anything, but I'm just hoping that the chemicals in the syringe aren't too carcinogenic because I am now the inadvertent in vivo test subject.

Another scary one was when I did a reaction that produces an equivalent of HCN.  While I was cleaning up, the flask slipped out of my hand and spilled some of the reaction sludge all over my bench.  I was really lucky since I didn't actually get any on myself, but I was just really shaken by the fact that I had lost control of this incredibly toxic material.  No harm done, but I had to go and take a walk outside before going back to lab.

Umm in high school chem we did sodium and water and the classroom blew up in flames.  Well, maybe not
"blew up" but a good portion of the room burnt down.

I tried to clock in early today for work (I started at 1:30 and got there at 1:28...) since I was almost late and the  entire management team caught me and lectured me for 5 minutes before I could go get ready.  So all in all I was pretty late for the first time ever.  

Oh and I've only screwed up 4 times in my entire career, and 3 of those times a manager was right behind me, and 1 of those times the store owner was behind me.  

Baaaaaaaad luck I tell ya.  Oh, are we still talking about labs?  

while i havent had any roller-coaster lab rides, i still have my share of annoyances

during one QA test, i was supposed to add ethanol to some solution. then i took a good inhalation of the "sweet vapour". my nose instantly fired up and i had heart pains for a week after.

Yeah, I know now of the dangers of manganese heptoxide, I didn't however, at age 12-13, if I KNEW the stuff was likely to blow up in a cloud of ozone and acidic fumes in my face at any moment, I probably wouldn't have kept it in a 2l glass bottle

Often, I find, that one of the worst parts of chemistry,  is those times where one needs reagents deemed suspicious by the drug-nazi governments, and where one is left waiting for several days, on edge, wondering wether what you get in the mail next is that liter of acetic anhydride you paid £40 for, or a customs McNastygram.

I REALLY hate those sort of situations, even more than physical dangers in the lab, at least you will PROBABLY only cause a lot of pain and disfigurement, or a hospital visit (for which you get yummy painkilers ), rather than the dreaded investigation.

Heh, I remember another incident as a kid now I think back, I used to be big into pyro, of the most obnoxious, teen wannabe "anarchist" sort, forever causing hell, at the time, I had a small arsenal of improvised rockets, and assorted explosive devices (none of which I do any more), and being inexperienced, I had under my bed, one 3 kilogram tub of a mixture of KMnO4 and sucrose, and one large container FULL of a chlorate/sulfur composition that at the time, I liked to fill pipe-bombs with, or empty CO2 canisters.

I had absolutely no idea of what can happen by autocatalysed decomposition of the chlorate into ClO2, from residual acid on the sulfur, which then sets off the resulting entire damned charge, luckily, nothing came of it, but I stored the bucketful of S/NaClO3 for several months, until I eventually misused it, looking back, I am lucky some of the s#*$ in my younger days didn't take off more than just the flesh of part of my hand