Unfortunately - though all data are still in the file when you just did a "save" - you can't make a "trim" undone once you did a "save", except you're a good QT programmer.
Apple should have some of those.
While editing in QT Player Pro you always should work with a copy of the original file and then use either a self-contained or dependent save. Dependent though just works for 3 or 4 levels.
Andreas