Every 2011-2014 3.7 or 5.0 (no GT500 has done this to me) mustang I have ever raced that had a manual transmission has done this (I race every weekend, and I have only driven my own car six times this year; people throw me their keys to figure out where they lose time to me). This will typically happen on high rpm, high throttle shifts.
As soon as I pull the assistance spring from the pedal, the problem is impossible to reproduce (though you will have about 1/8" of play at the top of the travel that the spring would normally hide).
2014 Mustang V6-Built to run TTB, CAM-C, and ESP
Proving to my fellow racers that the disdain that some GT drivers have for V6 drivers is unfounded, one results archive at a time since 2013.