Is following simple instructions so hard?

I am always amazed at the number of people who find it hard, or even impossible, to follow the simplest instructions.

Is it because they don't read the instructions?
Maybe it's because the instructions are actually not clear enough.
Or is it because they think they know better, or have a different way of working?

Whatever it is, it is very frustrating and causes me (and therefore, I'm sure, many other bosses, project managers etc) a lot of wasted time, either repeating, rewriting instructions or covering for the errors.

And yet through all this you must remain calm and make sure that in the end things are done properly. Email helps in this respect. So long as you are not confronted with errors face to face, then you can count to ten, and send an email encouraging the recipient to do it as per the instructions this time...