For example, a computer security expert can be an expert and making a basic mistake, perhaps unrelated to his personality but highly relevant to his field of expertise and a mistake that just about anybody could do but just because he is a security expert he especially should not have done it.
I don't mean stupid mistake or stupid error but I mean contradictory and almost similar to a double-standard.
Jeff teaches English but he couldn't spell "occurrence".
Or for example
George is a professional Microsoft Windows programmer but he doesn't know what alt+tab does.
Henry is a surgeon but he cut himself shaving.
I don't necessarily mean something like a police officer who himself is corrupt or a judge who is corrupt because those are not accidents or mistakes but deliberate.
Albert Einstein was a genius but couldn't remember his own address and entered the wrong house when going home.
(Perhaps the above Einstein example is not a great example because scientist could be absent-minded.)
I mean more like actually mistakenly missing the basics of your own speciality or making a human error that anybody else could do except you because you are supposed to be the specialist.
I think the classic example could be Bill Gates trying to show how good Microsoft Windows is and it crashes.
Or a news anchor, who should be a specialist in not making a word mistake, actually makes a word mistake.
Or a judge or an attorney, who should be experts in law and rights, wouldn't know something trivial about law and right e.g. unknowingly about that blackmailing is a crime.
I almost made such a mistake myself. I was an IT specialist and computer technician at a client's office and their boss was teaching himself the basics of programming and asking me about something basic. I actually got it right but I was not completely certain about something basic while actually being able to solve complicated and advanced problems for them.