Oh man, I know how annoying bugs like this are. It can be so frustrating! Have you considered using version control like Git? It would really help your peace of mind if you know you can "go back in time" and see all the previous versions, and what you changed. I'm not sure if GameMaker's file formats make this hard or not. GitHub has free private repos now, so you can get your code up in the cloud for backup, without letting it out of the bag.
Sometimes the only way I've fixed a bug is to see the diffs of what I recently changed...