I know what you mean and I guess many others have been in the exact same position. When it happens and I am having such a bad day to a degree that I can't even answer the simplest questions, I simply press 2, and move on. With thousands of cards in my decks I came to the conclusion that it would be a waste of time to push 1. Some days, or in some cases several weeks or months later, the same card will be easy (again) in case it was a mature card. And if you ask me, not every word that is in the decks, is necessarily worth to be remembered. So don't make your life harder than it has to be, by failing (mature) cards that you otherwise be able to answer in seconds, just because you had "One of those days." Exposure fixes it in the end anyway. :-)

Ah yes, those days happen. When that happens, I find getting all the reviews done works best (or as many as possible before you're bored). Spread them out if you must.   And like Kuma said, forget about adding cards.  As long as you're doing the reviews, that's a step towards retaining things, which is just as important as learning new stuff.( I believe)