Button problems: Hitting my Back Button Once Stops my Project Completely

Are you assigning all this code on frame 1? If so, if you go back to frame 1 you're piling on even more assignments and function definitions over and over.

You should assign the code in frame 1 and then start your while timeline on frame 2. Return back to that (e.g. gotoAndStop(2)) so you never re-run the same code again. Otherwise update the logic so it only runs once.

e.g. on frame 1:

if (!firstRun) {           // now on thsi frame this var exists forever and the code to assign never runs again           var firstRun:Boolean = true;             // I took 'public' off this as none of the other functions have           // "class" specific access modifiers.. add it back if you're showing           // a mix of class scripting and frame scripting           function main()           {                     start_btn.addEventListener(MouseEvent.CLICK, clicker);                     controlbtn.addEventListener(MouseEvent.CLICK, ctrls);                     creditbtn.addEventListener(MouseEvent.CLICK, credit);                     bacnbtn.addEventListener(MouseEvent.CLICK, back);           }           function ctrls(e:MouseEvent)           {                     gotoAndStop(10);           }           function credit(e:MouseEvent)           {                     gotoAndStop(9);           }           function clicker(e:MouseEvent)           {                     gotoAndStop(2);                     StartLevel();           }   function back(e:MouseEvent)   {   gotoAndStop(1);   } }

Now upon returning to frame 1 functions won't be redefined (erroring) nor will assignments be added to buttons that already have them.