That is not what I meant by dirty, that's simply a missed spell. By "dirty" I meant while mob uses a TP move, is casting or stunned and hence, despite landing, the spell will not stagger a weakness.
But technically, the spell hasn't missed. It can still land and do damage, but if the spell's added effect doesn't land for whatever reason, it doesn't get the proc. At least, that's what I've heard some BLUs say. No idea how accurate that is.

It's kinda like the inverse of when Blitzstrahl was a proc in Abyssea; where its stun effect would take effect before it would proc the mob, and therefore if the stun effect landed, it was unable to get a proc.