Modification visible on face-down exiled card that was perpetually modified by other effect
If a card is perpetually modified in a deck and gets exiled face-down, the modification is visible on the face-down card, giving information that should not be available.
Situation where I encountered this issue:
Board state:
* Bomat Courier
* Goblin Morale Sergeant
* Any other creature
Steps to reproduce:
* Attack with Bomat Courier
* Enlist creature with Goblin Morale Sergeant and conjure duplicate to library.
Results:
* If the conjured card goes on top of library, it gets exiled with Bomat Courier and Haste modifier is visible on the face-down card;
* If the conjured card gets deeper, the Haste modifier will still be visible if card gets exiled during later attack with Bomat Courier;
I'd assume similar situations can happen with other effects that modify cards perpetually and exile them face down.