Scooby,
I got one X marked next to a course that is a noncredit seminar class for a special program I'm enrolled in. Looking back now, I should not have even put it in my courses but its listed on my transcript so I thought that AMCAS would think I was trying to subvert the system if I didnt list it.
I emailed AMCAS asking them exactly what X and O marks mean; whether that means your app is on hold or if you have to redo your courses or send in another transcript. Havent gotten a response yet.
I thought about sending them a note thru the online "change notification process" feature but it states that its just like regular email so I figured I would avoid recertifying/resubmitting my app and just email them the regular way.
I will let you know if they respond.