Dropped something super heavy on my big toe last year, a month later the nail fell of. When it grew back several months later, it had like a horizontal groove on the nail. Illustration below, looking at the nail from the side.
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was freaked out, but it's slowly grown and it's pretty much normal now 
I smashed the nail on my right ring finger during football practice in high school. We were doing blocking drills and my teammate's facemask slammed right into the nail. It hurt like hell. Tore up the skin under the nail and a ton of blood started pooling up underneath it. It was throbbing with my pulse. The trainer heated up a needle and popped my nail with it. For the first couple seconds the needle was in, blood spurted out about a foot. Our trainer was new and had never seen it before - she screamed when the blood spurted. A few of my teammates were watching and they freaked out too. It felt great though, the pressure from the blood was starting to get unbearable. The rest slowly drained out. Had to do this once a day over the next few days until bleeding stopped, although it only dribbled out afterwards. Until my nail eventually fell off about a month or so later, I had to get it wrapped in this hard shell and a bunch of athletic tape for practice/games. Couldn't feel a thing through the padding, so that was good.
I ended up getting a huge infection in that cut. Since it was under the nail, nothing could be done to clean it. When I saw the doctor, he told me I was starting to get gangrene and if I'd waited a day or two longer to get some antibiotics I would've lost that finger (I blew off my first appointment with him a few days earlier because a family friend gave us Angels playoff tickets). It was so nasty while it was infected. From the tip until about halfway to the knuckle, it was swollen up to three time its size. The skin was bluish/green. Pus and blood continually leaked out from under the nail. Fortunately it didn't smell bad. One morning I overslept and was half an hour late to my first class. I knocked on the door, the teacher opened it and told me to get a tardy slip from the office. I showed him my finger and said "Sorry I'm late but I had to clean up my infection before I could come to my school." Teacher felt bad for me so it got me out of a tardy (tardies got us sprints and up downs at practice).
After the infection cleared up, my nail started getting really loose. It got to the point where I could completely lift it up. Imagine if your nail was a door that opened up and down and a tiny bit of skin at the base of the nail was all that held it on. One day I was playing with it like that in class and it finally came off (no blood, just fell off). My teacher saw and she almost fainted. I tried telling her it's not a big deal and it didn't hurt at all but she freaked out and made me go to the nurse. Nurse told me to go home, so I got a free half-day out of it. Over the course of six weeks or so the new nail grew in. It looks a little Fake Sugar Dick (WARNING, NOT THE REAL SUGAR DICK!) and different from the rest of my nails but not grotesquely deformed.
