Pretty much anything drawing (especially pixel art related) where drawing something is hard because it's testing my patience rather than my drawing skill.
One example is when I was trying to draw a pixel art sprite sheet, but I realized I need the character to be a little taller. I increase the sprite size in Aseprite, and tediously tweak 23 frames of the character to be more on model. I didn't learn anything from it, it was pain with unsatisfying gain