My coding question I give (in the occasion I am called upon to ask one to candidates) typically is not particularly unique, but practical - if someone wants to study for it, it doesn't really give them a notable edge given all the possible branching questions. The most notable tell if people did see the question prior though is if they jump right into talking about the problem without asking qualifying questions, implying that they are quite familiar with the problem & don't need me to qualify it & don't think to verify scenarios with me.

For my non-coding problems, I just create it from scratch depending on the position/needs & spend a bit of time navigating the scenario myself and store the question in my notes.

As to failing a question, failing a single question isn't necessarily a deal breaker in itself - it's showing a pattern of not meeting the bar that is. I may rate someone a 2 out of 4 if they didn't go into sufficient depth in a particular question I asked, but I probably won't stay in the way of hiring them if they did ok otherwise and that failure was just an aberration. Loss of integrity is perception that is likely to sour people on any upside of hiring though, and overcoming that bar is incredibly difficult - if someone is clearly rehearsed on a particular question and is dishonest about it, they're probably not getting a 3 or 4.