In your opinion, what is the most common violation of uniform regulations, and are you guilty of it from time to time?

Pockets almost always e7 and up

Being outside and not wearing headgear or having hands in pockets. The biggest of the violators are recruiters. I know I was a recruiter and I violated it every day I was always putting my hands in my pockets and almost never wore a hat.

My boots are not the cleanest

Hands in Pockets, Shaving, Hair styles.

I have to say Head Gear.
Indoors vs outdoors
Ranger / Tanker Roll
Berets in the field
Boonie cap rolled with the sting holding it up, or across the back of the neck. (AR 670-1 says under the chin, or tucked).

Women love the hairstyles(especially ones that interfere with the wear of the hat). I wont mention the long fingernails.
The Beret Flash over the eye, hanging between the top and bottom of the ear. You would think this was easy. Nope. Flash anywhere between the center of the forehead and the ear. Flap from the headband to flowing over the shoulder (never figured out how he did it, and I saw him giving the class on proper wear).
Starching in the shape, sewing in the desired lines.
Former Marines folding their cap, to pass for the marine corps hat.
Smarta$$ that I am, I would pull the flap over my eye in formation and mumbled Arrr!!!. When someone finally couldn't stand it anymore and asked what I was doing,, I would tell them..."It's Pirate Appreciation Day, and I am paying homage to my ancestors...Arrrr" They would laugh and I would fix myself.
Another time I got frustrated with all the different styles of wearing the hat, so I pulled the flash to center and Flapped it down to look like one of those golf caps. I walked all the way around my building, in and out of the barracks, through the hospital and back out the door. Only one person questioned it. The next day, to my horror, everyone started wearing it that way. I gave up after that.

A close second would be starching the Uniform. ACU is not supposed to be starched, but guess what? They do. Even the BDU service tag says on line 7 "do not use Chlorine bleach or starch". I don't know about the other services, but AR 670 clearly states: "Shall be maintained in accordance with the service tag", "The commander is not authorized to require starching the Uniform, except for special occasions." However, try to pass a board or inspection, without. Good luck.

Hands in the pocket, most def.

I had an 06 from another base, extremely overweight and very resentful of the families and bond within the unit, it common for her to wear unauthorized uniforms on Fairchild and even though 99 percent of the enlisted complained about her our higher command refused to enforce the dress standards, shes the same one who refused my the chance to be 1st Sergeant because I could have everyone in my unit in less than 4 hours completely ready to deploy, she told me I undermine her authority, we even get into the racial stuff she said.

As an airman, I thought the blousing procedures of the BDU's were the most ridiculous thing I'd ever seen for a military uniform! I used to blouse BDU's Army style and never got pegged for it! It was just a better and neater looking appearance! This is the only item I'd ever bend until they came out with some strict guidelines toward the end of my career!

Hands in pockets. We have all been guilty of that one time or another LOL

When I was in AIT, I was caught having my hands in my pockets. When I was in Germany I almost didn't get into PLDC because of my Military ID. My picture had me sporting a flat-top haircut.

As a young LCpl I would wear white socks instead of green with my cammies at times. This only works if you dont sit down. The boot bands ride the trousers up and all is shown. After I got yelled at I soon found out I was a fast learner. There were guys I knew, who had hairy chests, not wear dog tags because they yank hair out all day. Semper fi