Coworker implies paid work, and therefore they are not amateurs. They very well may make the same mistakes, but those mistakes would be professional mistakes.
Why this level of pedantry when the meaning is absolutely clear? A professional can make an amateur mistake. This makes perfect sense. That isn't implying the professional is actually an amateur, but that he made a mistake that an amateur would make.
For some added pedantry: aren't all the mistakes that a professional might make, also ones an amateur would make?
In fact, it seems like an amateur is likely to run into all mistakes more often, thereby making all mistakes amateur mistakes; unless there some class of mistake that amateurs are better at avoiding?
