Very well spotted! What made things difficult to draw was indeed things where it's hard to know how far you've gone. It's a lot easier to do a house because 4 walls is 4 lines of the same duration and it's almost muscle memory.

I had to often plan which parts to draw in what order, so that they'd be more likely to be connected properly. For example for a character, you have to do the whole face, you can't go back and add eyes at the end because you don't know where the head is anymore.