I've been to a few training professionals with different ideas of which marksmanship fundamental is most often messed up. Defining the fundamentals as sight alignment, sight picture, trigger control, and follow through.
For example, in the Tom Givens class I believe he said it's almost always trigger control. If I remember right, Awerbuck said follow-through was the most common cause.
Sometimes when I'm shooting, I get on the trigger too fast, before the sights are lined up or the front sight is settled where it needs to be. I can sometimes even guses where my miss will be because I wasn't managing the sights but I recognized the misalignment as I was breaking the shot. Sometimes I don't remember seeing the sights at all. So in these cases, I believe sight alignment and sight picture are the fundamentals I'm screwing up.
I also have plenty of instances where I know I'm not managing the trigger (or keeping the gun in alignment until the bullet leaves the barrel, which like rhino, I've come to lump in to "trigger control"). Sometimes I can feel myself do it, some times I can't. Usually when I'm missing and I can't identify the cause, I assume it's a trigger control issue.