I'm not sure why there's so many attempts to make The Catch into something so exotic that's it's almost impossible for it to come up. I'd rather see The Catch be something that likely will come up in play (especially for PCs with The Catch) and instead try to make the creature/opponent more interesting and have to think through things rather than just rely on a near-impervious defense from a super-obscure application of The Catch.
This is a thread for Catch ideas, which implies exotic ideas, as opposed to the mundane. It's easy to say "The Catch is Sunlight/sugar/glass/blond hair/etc and you have to deal with that in play." But that can get boring as well, to the point where you might as well not have those powers.
Also, the spice of the exotic can keep a game interesting. Fairies and cold iron? Been there, done that. Black Court? Murphy called it living the cliche for a reason. But a big McNasty from the Nevernever who can kill with its mind... and is only vulnerable to the application of obscure mathematical treatises? That'll be an encounter that the PCs will remember for a long time.
And for other considerations, sometimes the weird Catches make sense. I know that a magical assassin with an easy to exploit weakness is going to have a short career, so in cases like that, an obscure/exotic Catch makes sense and adds tension (and, if handled right, drama).
