When making drum lessons, I made a huge mistake in the prototype.  Well, what I mean, is that the basic method of doing things has to work - or you'll end up having to do everything over - which might involve many copies being redone!

For instance, the drum lesson software I was using didn't render notes correctly. Well, actually, the errors weren't enough to throw people off!  However, they were still "off the mark".  I mean, I have a professional project going!