Sometimes when the changes I make to the code don't seem to have any effect on how the computer behaves, it turns out I was just editing the wrong file. Everyone makes this mistake sometimes Also, errors that point at commented-out code might be syntax errors, so it could be a missing punctuation mark.
Another option is that if you're using a version control system like Git or Hg then you can compare your current code to older versions and see exactly what changed. This can help narrow down the search for what caused a problem.
Finally, if re-typing the code in the exact same way fixes the problem, then there must be something else that's different. Maybe the computer can help you track down the difference: