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

Uniforms that obviously do not have proper fit. Some are way too loose. They dont look or fit properly. This includes uniforms that fit way too tight. Theres a proper military appearance, a way uniforms are designed to fit. I was accidently guilty depending on where I bought them. One place sold uniforms that seemed to shrink a few sizes smaller, so quit buying from them. As supply, I offered to order better fitting uniforms for soldiers. Oh and also, field caps. For heaven sakes make them look sharp the best you can. Don't just throw them on your head without a second thought. Had to get that off my chest haha

The obverse of the question, i.e. the least common violation of uniform regulations may have been in May/June of 1970. By the end of a 71 day patrol several, to include me, of our infantry platoon we wearing parts North Vietnamese Army uniforms, as ours had rotted off our bodies.
I was surprised to see photographic evidence of said uniform adaptations in "Blackhorse Riders" on page 4 of the photographs. This took place in Tay Ninh.
At the end of our 71 days we were ordered to clean up, shave and put on new uniforms, having been provided with soap, water and new uniforms by a UH-1. We refused the order, stating 'take us the way we are'. It flew off stating there would be no extraction until we did so.
We complied, a small gaggle of slicks returned, at which time we were photographed and lifted out.

I will agree that the hands on the pockets is by far the most frequent violation. However when I was on active duty I would occasionally misplaced my hat

Soldiers here in Fayetteville NC, from Bragg going to off post Dr. Appts. walking from their cars, to the building, across the parking lot, in MCUs, hands in pockets, and NO COVER. I look at them and I see the AA, and I say to myself, so that's how the Eighty Deuce rolls now?

My uniform voilation occurred when I was active duty and had just been assigned to a new Squadron. During Commander's Call is missed hearing my line number for promotion to Sargeant. My First Sargeant called me out on it the following day, I was out of uniform and he said either have Sargeant's stripes by the next morning or be written up for being out of uniform. I had Sargeant's stripes the following morning.

Back in the day, I was guilty of not wearing the uniform wool socks. They itched terribly. I wore either dark brown or black. Same with glove inserts. I was never really "caught" with those.

But ... I was a photographer, and when working often had my cover in slightly the wrong position because it got in the way. Also, I couldn't shoot with those heavy gloves on, so even if they were the uniform, I either wasn't wearing any or had on thin black ones. Occasionally, I'd get a "correction" but if I was allowed to explain, it usually wasn't a problem.

The biggest corrections I made were unbuttoned pockets or pen in pocket ... usually an officer lol ... when I was taking their photo.

I was a bit selective about a "Ranger Roll" in the patrol cap. If you were Infantry, and I knew it, you would get a pass. (Mainly because I was also 11 series) I would make on the spot corrections on obvious violations to 670-1, but I wasn't a "Reg Nazi".

Hands in the pockets

Being retired military I do attend many veteran specific or ceremonial events where medals, uniforms worn by military retired or veterans present. The most common issues post active duty military use of the Uniform is mixing of the Uniform with grooming standards, mixing of civilian and military gear or clothing, medals/ribbons on civilian clothing, getting order of presidence out of order, and wearing such at events where perception of endorsement of the DOD or service branch of the gathering is in the grey area, to but it kindly, partisan rallies, beer gardens, etc.

Having discipline is important but some of the biggest offenders in peacetime turn out to be fierce warriors in combat. Chesty, MacArthur and LeMay could do what ever the hell they wanted because they were true warriors.