SWF works with 9.0 r45 and not 9.0 r28 and below

As the title suggests, I have a SWF that is working with the very latest version of Flash; however, for some reason the Flash doesn’t play in less than 9.0 r45, meaning those with 9.0 r28 can’t view it!

What on earth did Adobe change from those two versions?

As you can imagine, this problem is incredibly difficult to debug.