Especially if you used a 747. 
When I was about 7ish, I let a neighbor girl go at my (very fine, very straight) long hair with a crimping iron. She was perhaps 10. The first hank just came off my head entirely as she burned it away. What I find more amusing is that I continued to let her ‘crimp’ the rest of my hair after that.
When in college, I bleached the everloving shit out of it for a year or so. It was loads of fun, but I looked like a cheap hooker, and spent way too much money on conditioners and pixie cuts to keep it somewhat reasonable-looking.
One time I very nearly killed my shower drainage. My hair made it through fine, but someone told me that if you had very oily skin (which I do) that you could use a clay mask on your scalp to help dry the oils up. So I went out into our yard, and pulled up some of our bright orange-red clay subsoil, and proceeded to apply it to my head. I was a little shocked that it dyed my scalp bright orange (and even more shocked that you could clearly tell through my hair) but when the clay grime began bubbling back up out of the drain, I knew I had bigger problems.
Bleached my hair with actual household bleach. At least we had the sense not to just let it soak, just poured it over and washed it out. Still affected the texture some.
Went through a phase where I shaved off an inch or so of hair above and around my ears. It was a less extreme version of a style quite a few other kids had (guys and girls) that involved shaving everything from a few inches above the ears down around the back of the head, and wearing the rest in a pony tail or small braids.
Worst thing done to my hair by someone else was when my mom brought us in for a very rare salon cut. She told the stylist to cut my mid-back lenght hair short and turned her attention to the younger kids. The stylist proceeded to cut my hair very short indeed, as she later told my mom, she thought I was a boy! I was kinda lucky at the time the style for boys was on the long side, but still unmistakably a boy’s cut. I hope she didn’t pay for that one.
In HS and college, my hair was down to my belt (guy here). Last year, about 30 years later, I decided to start letting it get long again. I could only lie to my self about the appearance for 2 months, then back to my customary old white guy nondescript style.
One time when I was about 12 or so, I wanted to dress up like a 1950’s “Greaser”.
So I got the blue jeans, with a white T-shirt that had an empty pack of cigarettes rolled up in the sleeve. And for my hair, I used Vaseline to slick my hair back.
Big f’n mistake, it took me a week before I was able to wash all that stuff out. At school, my classmates kept asking me “Did you go swimming last period?” (We had an indoor pool there)
“Uh, no my hair is not wet. I’m just an idiot.”
Sun-In when I was a teenager. Not a good idea. That’s what started my 25-year affair with real dye.
I finally got curious a couple years ago, what’s my hair color, really? And I knew there were grays but not how much. So I dyed as close to natural as I thought and let it grow out. Turns out it’s even darker brown than I thought, once the porosity from the dyed parts grew out, and I like the grays. It’s been so nice not to have to dye it every 6 weeks!
This was for Halloween, not just because. Can’t believe I left that part out.
Guy here, with dark brown hair.
About 12 years ago my workplace got involved in one of those “head-shave for a cure” money raising things after a co-worker lost her baby to cancer.
About 20 of us were up for either a head shave or a (spray-on) colour. I’ve always had pretty short hair so I figured the head shave wasn’t much of a sacrifice and a spray-on colour was a bit of a soft option so I came up with the idea that I would get my hair bleached if I could raise over $200.
I let my hair get longer than it has in years and on the day went to the hair-dresser that was assisting with the event. The whole thing was an experience as I’d never had my hair coloured in any way before (I was about 34 at the time).
The first application of bleach (with the big rubber head-condom) left my hair a nice canary yellow so a second application was done. I stuck that out for about 20 minutes before the burning on my scalp became too much.
Still, I got a good white bleach out of it. Everyone except my mum thought it looked great.
I swam every day for two months straight last year, completely ignorant of the obvious fact that it was turning my hair into straw. I had hair to my waist. By summer’s end it was to my shoulders and still needs more cut off. This year I’ll be a smart girl and get a cap.
Spiral permed my bra-strap length hair. I ended up with a horrible knotted mass at the nape of my neck, and it took almost three years to get rid of it all.
I trimmed away at a full beard, leaving an unusual mustache that went up my cheeks and a goat-like goatee. Then dyed all of my hair a light sandy brown. I did this in order to fit the part I had in a play in the community theatre.
When I was about 10 my grandmother took my mom and I to her (male) hairdresser. This being the 80’s I said I wanted it “parted in the middle and feathered back at the sides.” Well this guy proceeded to do the exact opposite and gave me a PIXIE CUT. I was a very mousy child and intimidated by this great big guy who had been my Nana’s hairdresser for years and who she adored so I didn’t speak up at the time. My hair is naturally wavy so you can imagine how awful it looked. So there I was later trying desperately to make my ruined hair look cool and there was a cowlick at the crown that just wouldn’t stay down. So I cut it off. Just snipped off a big chunk of hair from the top of my head. Then as it grew back it would stick up again so I had to keep trimming this one-inch spot of hair, until I finally realized how utterly stupid it looked and had the patience to grow it out.