Any similar stories?

Got back grades for my previous semester in complex analysis a while ago, but I've still been bugged ever since. The grading consisted of homework, midterm, and a final. I was very much on top of the material. Come the midterm, I finish 3 out of 4 problems swiftly, then the last one seems very difficult. I figured, heh, no one else is getting this, I'm probably fine. Grades for the midterm come out and everyone seems to have got it. Turns out, in the TA led section before the midterm, they showed a trick to do the exact problem that appeared in the midterm... and I did not attend that TA section... Come the final, I do very well (much higher than median), and should have landed comfortably in A territory, had it not been for the midterm.
In the end, I lose an entire letter grade because of a technical trick I was not aware of on the midterm. The moral of the story is to attend the TA sections, but I want to know if r/mathematics has any similar stories so I can feel better... Any stories where you feel you understood the material like the back of your hand, and yet, choked somewhere?