I worked as a TA at my university for a while (under-grad TA too ... was a blast, me and the prof got along really well, and I knew the material extremely well and could teach) and the hump mentioned is true. I once used to believe that everyone could learn how to program, if you can write down steps for making toast in the morning you know how to program, but some people just couldn't get it.

There was some mental disconnect that didn't allow them to grasp certain key features required for programming. No matter how you explain it to them, no matter how you tried to change the model to help them. I spent a lot of time with the professor I was TA'ing for trying to understand why, trying to understand what we were missing that could help these students understand. Stuff like scope, pointers, references were some of the difficult topics, which to me come almost naturally they felt it hard to wrap their brains around.

There were students that didn't get it but they could fake the knowledge required and could get the end result but not understand why that was the end result. They did okay on the programming questions, but when it came to knowledge of what was going on they couldn't explain it.

I felt terrible having to fail students, I felt even more terrible when I could see on their face that they had tried their absolute best. I don't think I could go into teaching unless I taught uper-level classes so that I didn't have to deal with the great disappointment and filtering of students. The look on a students face as he was sitting in my office during TA hours and he still wasn't able to grasp the concept of pointers made me feel really sad. I don't think I can deal with that again.