I have a pronounced case. I can hold a cup of water in the hollow of my chest (when I’m on my back of course.) Totally sucked going through puberty, let me tell you.
A few years ago there was a boy on my son’s swim team with sunken chest and I felt so sorry for him when he had to stand up on the blocks. Then I suddenly realized his chest was just like mine and I was on a swim team too. Except for a very small set of mean kids, I managed to survive without too much hate.
My family laughs at me because I do the same thing! I don’t do it on purpose, it just happens.
More sinister.
Ok searching for nostril flaring on the Internet found this thread: Nostril flarers - report in! - In My Humble Opinion - Straight Dope Message Board and an older thread, linked inside, similar to this thread on quirks/mutations. Looks like everyone can do it. I wonder where I read that it was rare.
Everyone can learn. It’s connected to a throat thing, singers learn to do it. Do it while singing a simple song, and pay attention to what happens in your throat. Feel it? ETA - People who have difficulty flaring their nostrils can, conversely, figure out the throat position by working with a mirror and then do the thing with their throat to flare their nostrils.
I don’t have proper nerve sensation in the soles of my feet, and neither does my aunt. We don’t really feel pain at all, and don’t feel tickling. I feel pressure a bit, so I can usually tell if I am being touched. I also feel heat and cold a bit, I feel the burn of very hot sand. But if I walk bare foot I tend to slice my soles to bits because I just can’t feel when I hurt them. I’ve always been able to run on gravel, though it’s not a good idea.
I once climbed a fence and heard a spike pop through my shoe. It felt like I had peed on my foot: that was blood. I took off my shoe and saw that the spike had gone right through to the bone, but I couldn’t feel it at all. The doctor kept asking: “you really can’t feel that?!”
After reading through this, I realize I’ve got more weird goin’ on than I thought I did. I was born without four of my molars; the baby molars stayed in until I was a teenager and they pulled three of them to make room when the braces went on. The fourth came out in a Sugar Daddy caramel pop when I was about 11.
I can flare my nostrils easily, and I can wiggle each ear independently. I can also arch my left eyebrow at will, which has been a huge source of ire and amusement among my friends and acquaintances all my life. Occasionally people would get really pissed off when I did it. 
In the Toe Department, my pinkie toes are “side toes.” the nail is pointed directly to the side, rather than top. My aunt was thrilled when she noticed it when I was a child; it’s a family trait apparently. It doesn’t interfere with being able to pick things up with my feet, though, which I can do also. Once you see video of people without use of their arms doing all kinds of things with their feet though, you stop being impressed with picking up pencils. 
Dung Beetle, my tongue looks like that when I get slightly dehydrated.
Oh! Hubby had almost another full set of adult teeth (SHARK TEETH) that had to be removed surgically.
Of course, the joke was on them when he took a body check into the boards and lost his front teeth in the process. Maybe they should have held onto those spares…
Been there, done that. Got off a conference call with our office in Dublin, and a coworker who didn’t know me well commented that he hadn’t known I was Irish. (I later found out that the guys on the other end of the call thought I was Irish, too, so at least I didn’t offend them.)
Do you ever get stuck in an accent? I did a Brooklyn accent for a role last year, and it kept coming back on me for hours.
(This accent stuff isn’t really a physical quirk, though. At least, I don’t think it is.)
Could be that, I’m chronically dehydrated.
Yes. I spent a day with a nice guy from Tennessee - it took a good two weeks to eliminate that. I didn’t even notice, but my family and friends sure did.
I didn’t think it was a quirk or special either, until I was chatting with some other theatre types and they were lamenting trying to get an Irish accent right…I guess it’s not common with all actors.
I can fart like a whale. Hirschsprung's disease - Wikipedia
A few more…never knew this place was such a sideshow!
I have similar to the talon cusps of another poster, but on the forward side of my upper bridge, only on one side. Where most people have a nice pink gumline, mine is a knotted gnarly twisted mess of protruding roots, looks like I’m broodmothering a bunch of spider egg sacks. Fortunately, it doesn’t show when I smile, so I only freak out my dentist.
I used to have one trapezius that was significantly noticeably larger than the other, due to many years of habitually lugging my textbook bag on the same shoulder. I was forced by shoulderopaths (or whatever that field is called) to alter my habit in my late teens or risk spinal problems. Today, I’m fine and symmetrical.
One, and only one, of my eyebrow hairs is the gauge of piano wire. I pluck that insectile monstrosity whenever it shows itself.
Scapulamancers? No, wait, those are the guys who stick shoulder blades with hot pokers to predict the future. Never mind.
It’s no physical thing, but I have aspergers. With aspergers, there are plenty of quirks I have, too many to name! They are more like ticks.
Hmm I read this article almost 10 years ago, and it made me think waggling individual ears was rare too, yet many people here seem to be able to do it. http://discovermagazine.com/2005/jan/physiology-of-facial-expressions/#.UwQCSngazCQ can anyone do a fake real smile like in the article?
I can flare my nostrils, flex my scalp, crack my elbows like knuckles, and do a trick sideshow freaks do; make half my face smile and the other half frown.
I’m an actor, and I suppose positive reviews I have had suggest that my fake smiles are convincing? My facial muscles are pretty trained, I think. I can raise one eyebrow, that sort of stuff, but can’t do Enola’s simul-smile-frown.
I suppose you might say that as an actor the fake smile uses emotion as an impetus (in Stanislavski’s method). On the other hand I trained in a Lecoq school, which is movement-based. IOW emotion would derive from the experience of the movement of my facial muscles (that’s not what I do, I would more usually work from larger physicality, but just thinking it through).
Actually, thinking it through like that, I’m not sure that the divide between fake and real is an entirely useful distinction. We know from research that the act of smiling makes you happy. I think the muscle memory of raising the corners of the mouth will quite easily push the muscles around the eyes into action, but does that mean you are really smiling because you are now happy? Is it just muscle memory? What if the muscle memory makes you happy? 
OK, how many of us just tried it? 
I can crack my knuckles for both hands just by making a fist. This was a pretty awesome talent in 3rd grade.
I have one toe thumb (clubbed thumb, brachyldactly, murderer’s thumb). The other is normal.
The top of my butt crack is crooked. Just the actual crack. About half an inch. Just kind of curves to the right. There is no underlying structural/skeletal reason for this. Just a weird curve in the skin.
I have been teased endlessly by my siblings for both.