Sometimes after I write an ffc script and load it into a slot, it doesn't run at all when the selected enemy is onscreen unless I'm starting from a new game. Most of the time, it works just fine; but every once in a while, it doesn't.
In other cases, I have things which control the enemy's appearance; size, and so for which I alter and are control from within the ffc script but when loading the game to test, remain unchanged.
Strange, those aren't things that should require a new game. I wouldn't rule out some global variable weirdness, but usually that's a lot worse. Maybe there's some interaction with other scripts somehow?
These are what I put in the std.zh stuff for 2.6.
It's missing Misc[15]. But unless the handling's changed since 2.50.2, you can actually skip that, since Misc[] is cleared whenever Script is set.
And don't forget that it'll fail if an FFC calls it on itself, because it can't set its own script. It should probably call Quit() in that case.