What is so difficult to understand about "clear and obvious error".

The standard of our refereeing is at a pretty low level, doing it live is a difficult job but watching replays is a lot easier. Kevin Friend didn't have to decide whether it was a penalty or not but to decide if Mike Dean had made a "clear and obvious error" the fact that it took to minutes plus to make the decision says it all.

It's not brain surgery, is it.