When you Over Plan something, you make multiple plans over the same day or the same time (usually because you forgot about previously made plans). This can often lead to friends being angry with each-other, and sometimes to end of friendships.
Friend 1 - Hey dude, can't wait until tomorrow! Avatar's supposed to be amazing!! Imma get a huge tub of pop-corn!! :D
Friend 2 - Oh, sorry man, I can't make tomorrow. I'm going out shopping tomorrow with someone else. How about Sunday?
Friend 1 - THIS IS LIKE THE 5th TIME YOU'VE DONE THIS!! I've already booked the tickets and everything!! You really need to stop Over Planning things dude!!