Setting: The starship USS Odyssey.
Situation: after a briefing about an upcoming away mission, one of the officers assigned to the Away Team asks the Ship's Counselor (while both are still on the Bridge) for advice on a problem he is having with a fellow officer who has also been assigned to the away team. Officer #1 has diarrhea of the mouth--sharing way more details of the conflict than the Bridge officers need to know, and the Counselor hauls Officer #1 into the conference room (adjacent to the Bridge) to discuss Officer #1's problem in more privacy.
Once the situation is fully explained to the Counselor, the Counselor reads Officer #1 the riot act for being highly unprofessional. As Officer #1 and the Counselor calm down, they suddenly realize that the Captain and the second officer never exited the conference room after the mission briefing ; they are still there, hearing everything. Both the Counselor and Officer #1 have egg on their faces.
Granted, this isn't an entirely realistic situation--Both the Counselor and the young officer would have seen the Captain and second officer, and the Counselor would have immediately taken the young fellow to his office. But, because this happened in an email RP, in which there are no visuals except written text, this detail was overlooked, and it made a wonderful capper to the Counselor's frank discussion of views with the young officer.
I wrote the counselor's part of the above story, and this vignette has gotten me to thinking about when it is a good idea to allow characters to make mistakes. This particular incident works so well because I can use it to illustrate how profoundly abnormal my character's telepathy is--He literally did not notice that two other people were in the room because they were non-telepathic aliens. Thus he did not detect their mental signatures until his attention was drawn to them.
Under normal cirmstances--If I'd been writing the entire story and had remembered/been aware of all details--This disaster on the Counselor's part would never have happened. He would have seen the Captain and Second Officer in the conference room and would have immediately dragged the younger officer elsewhere. Because the detail was overlooked, we got to have a wonderful scene in which the counselor had his say and now has to eat humble pie for unintentionally doing the exact thing he was trying to prevent.
It bothers me a little that I would never have thought to have my character so completely miss something so evident, even though it works perfectly with the background I've developed for him. I've had my characters make mistakes before--my character Aerden made a doozy of one while training his healer apprentice in a Pern story I once wrote. But having my characters make mistakes which will advance the plot is not usually the thing I think of while writing. Instead, I am usually asking myself, "Is this the appropriate thing for him to do?" Character missteps on my part more usually arise frm my ignorance than from my planning. I think this is not a good thing.