Update: I was copying every code sample by hand for 2 months before I noticed my bug
I spent about 6 weeks stuck on a simple to-do app because the delete button kept removing the wrong item, and I kept rewriting the same loop over and over. What tipped me off was when a coworker at the store glanced at my screen and asked why I retyped examples instead of just reading what each line did first. I had been copying whole blocks into my file without knowing what half of them meant, so I could not spot that I was passing the index wrong. I deleted everything and rebuilt it line by line with the docs open next to me, and the whole thing took 3 hours. Has anyone else caught themselves just pasting instead of reading, and what made you stop?