As the title suggests, sometimes you are in front of a word and you guess it correctly but know that it's not yet stuck in your brain and know they require extra attention. There are words like that, you can find them in a context of learning, but then when comes the time to use them they are gone. So what I do personnaly is that I swipe them like I failed in order to have them back quicker, but they don't necessarily need to pop up as soon as new / failed words, and it feels unnatural to swipe to the failed deck when you got it, compared to the ones you don't have a clue. It would be nice to check something on the card to mark it as "hard", for these situations of "I know this one but it gives me a hard time or I'm always unsure". So it would be more frequent than regular correct guesses, but not as much as failed ones. These words just need extra attention for a while :)