Hi Kim, Welcome to the Disney Parks Moms Panel!
If you miss a
FastPass+ window, there is no penalty towards the other passes scheduled for that day. I've been there, and while it's not the best feeling in the world, mistakes do happen - previous experiences take longer than expected, you get lost (or side tracked shopping), you accidentally sleep late and miss half of your morning ... all of your classic vacation fails.
Unfortunately, the pass to the missed selection is forfeited and does count as one of your initial three experiences; once the window has passed you will be unable to reschedule it or make any more reservations for the day until you've completed your other two reservations. However, once you are able to make more selections you can re-choose the attraction you missed.
I do have great news, though: Disney has made amazing technological strides to improve your vacation and minimize the chance of the dreaded missed fast pass window. Back in the way olden days of 1999 - 2013, fast passes were printed on paper tickets. You could only hold one fast pass at a time, and if you missed your window all was basically lost.
Now thanks to FastPass+ and the
My Disney Experience mobile app, if you realize you're not going to make it somewhere on time, you can pull up the app and reschedule an alternative time for the attraction in question from anywhere: from the monorail or a bus, while standing in a queue of a current attraction, even from your bed upon realizing that you overslept. You can even cancel or reschedule a FastPass+ reservation minutes before the scheduled start of your window without penalty of losing that selection.
I hope this helped, and don't forget to set two alarms!
~Kristen