Also, a general one: As soon as you pick up an item, spell, weapon, ability, specific unit, etc. 98& of the upcoming enemies are immune or highly resistant to it.
I noticed it again most recently in Final Fantasy 6. You learn all those fricking Espers and by the time you have the debuff-spells at your disposal, none of them work. Sleep, Stop, Stone, Silence, Confuse, even Slow are entirely pointless. If the enemies are weak enough to be affected, they die quickly enough anyway. Out of frustration I even resorted to exploiting the Vanish-Doom-Bug a couple of times (damn Sneezing Brachosaurs).
But it happens everywhere. Oblivion: The entire Illusion school is useles until you've mastered it, because everything is level-dependent and you can never cast anything useful for 1 or 2 levels above you.
Sometimes it's the other way around. You crawl through a whole dungeon of things that are immune to X, at the end of it you get the "X-Ray" (heh) but then everything is immune to Y, Z and Interrobang and X rarely appears anymore.
WTF, devs!???!?