I'm not talking about forgetting to do something, or taking a moment to bring a certain word you want or a name to mind. Nor am I speaking of trying to learn a very difficult new subject like Latin or quantum physics. I'm talking about not grasping something that should be common knowledge to any average adult.
It can give rise to some very humorous situations or be the source of extreme frustration. I've experienced both consequences of interaction with this type of individual, but my mind fails me now to recall one of either.
If you have a humorous story to relate regarding this subject, I invite you to share it here in the comments.