I've got two.

A lab freezer's seal broke in the middle of a humid Michigan summer, so everything got encased in frost. In the process of chipping away the frost, the ink on many of the labels rubbed away, so we essentially had a bunch of mystery flasks. One such flask had a septum that was stuck really tight. When I yanked it out, the recoil caused some of the mystery liquid to splash onto my mesh shoe. Within a couple minutes, my foot started stinging. We later identified the contents to be acetyl chloride, so it was probably reacting with my foot sweat to make acetic acid and hydrochloric acid. I took my shoe and sock off and rinsed my foot in the lab sink.

I was putting sodium hydride into an empty round bottom and a good bit of it got stuck to the ground glass in the neck. Genius that I am, I turned a nitrogen line on with low flow thinking I could blow it into the flask. I didn't realize that the nitrogen had to go somewhere and the only place it could go is back out, blowing NaH all over my face. There was very much safety-glasses-unless-there's-an-inspection culture at my old university, but I was never more thankful that I made it a personal rule to wear splash goggles. Would not have liked for the moisture on my eyes to bubble off.