edge case

noun3

1. •
   A problem or situation that occurs only at an extreme (maximum or minimum) operating parameter or under other unusual operating conditions.
2. •
   Any extremely rare situation.
3. •
   Any unexpected input or user behaviour.

   • The first thing to do is analyze the situation and determine whether the need to provide additional use is an edge case, or if it will become a common use-case scenario. — 2010