Fourier Visualizations
In college, I took a CS elective that was shared with the electrical engineering major.  It was taught by a PHD student, and was 90 minutes twice a week.  I believe it was something along the lines of wireless technology.The first day, the teacher walked in, wrote an equation (FFT) on the board, and spent the next 90 minutes manipulating it into different forms (from his in-hand notes), with no explanation of what or why we were doing so.  Occasionally he would stop, say something along the lines of "and you can convert what we have here to this..." and write another equation on the board, and continue "which is one of your homework problems for next time."The next class was identical.  When the third class started the same way, I walked out 40 minutes into it and dropped it.  I had zero incentive to continue, the teacher hadn't explained at all what the purpose of what we were doing yet.  It had literally been us writing down his math notes for three straight hours while he explained how the mathematical manipulations worked, but not why it mattered or how it was relevant.I've vacillated over the years between being upset with myself for abandoning that class, and not "getting" fourier related things, and being mad at the teacher at being so horrible at teaching, at least for the portion I was there for.This is simple.  This is obvious.  Something like this would have gone a long way towards explaining what the fuck I was doing.  I'm no longer upset at myself.  I am doubly upset at that teacher now.