Here was my situation:
I created a tooling plate with an array of drilled holes for dowels and screws. The tooling plate exceeded the travel of my mill so my plan was to slide the part. Due to this, I needed to individually select holes for drilling that were within the envelope of the mill for that setup.
The problem became after selecting the 250th hole, I accidentally selected a point on the side of a hole. There was no way to deselect that point, so I had to cancel all of the points and start over again.