I've been a TA for several different labs at three different schools, and grading is usually the most disheartening part. The lab I was a TA for during the first half of the summer semester (one of those compressed courses) had a "practical lab" in the middle and at the end of the semester--in which students did a lab they'd done before, only this time independently in a foreshortened format--and I just finished grading a passel of them. (I think that's the official word for a group of such things--a herd of cows, a flock of geese, an unkindness of ravens, and a passel of practicals.)
 
Whenever I grade something, whether it's a regular lab report or a practical, I retrospectively realize things I should have said to make the concepts clearer or to elucidate what work was expected. Sometimes, I feel as though I've harped on a subject to the point where it must be annoying, but it doesn't percolate through to what I see in the lab reports. Perhaps I'm just repeating the same ineffective way of saying it (I think this is likely the case). Something about the way I'm attempting to communicate is not reaching people.
 
Sometimes, I feel I can say with Obi-Wan, "I have failed you, Anakin. I have failed you."
 
This leads to a quandary when grading. If a whole bunch of students make the same error, I think it's incumbent upon the teacher to reiterate the concept, lest the error become an entrenched mistake. On a summary exercise such as a practical lab or a final exam, however, there's no chance to go back the next week and say, "Here's what I really want you to do in this situation--look at this example on the board." So many of the little misconceptions I never properly addressed come bleeding out under the pressure of the summary exercise. I have to think that if a fairly large percentage of students made similar errors, I must not have explained the relevant concepts thoroughly enough. How do I grade fairly in that case? To follow along with the Star Wars parallel, Obi-Wan missed some rather glaring deviations of Anakin's ideas from mathematical possibility the Jedi Code, but Obi-Wan, having grown up as a Jedi, may not have realized the underlying assumptions about how the universe works which Anakin had picked up in his different training environment. Still, he was right to put the smackdown on Vader for graphing things weirdly killing, like, everybody. What's the grading equivalent?
 
I know I, when in student mode, subject my professors to this, too. Especially on exams, I do ridiculous things and come to bizarre conclusions which I'm too uptight at the time to recognize. There have been a few whom I feel I'm letting down when I don't do well at the subject matter. You can tell they really care how students are faring with the material, and it makes them sad that we don't get it. It just figures that struggling students make the good teachers sad and don't bother the bad teachers at all (since those don't care all that much about how students are doing). Still, for all the "Aw, shucks, they still don't understand" sadness, I would rather grow up to be a good teacher than a bad one.