Confusing instructions: why does it happen?

I recently received the good news that my book is going into its second printing. This gave me the chance for corrections and clarifications to make the instructions more accurate and easier to follow. It also led me to think about the various causes of problems or confusion when someone tries to follow instructions.

For starters, sometimes problems arise when, in our eagerness to get going, we fail to read the introduction or notes that contain vital facts, and plunge right in. When I started making collapsible baskets, I wondered why my cutting angle was always different from the one recommended. Three years later, I noticed, for the first time, the sentence that clearly said that when you enlarge or reduce a pattern, you have to change the cutting angle. I still don't know how I missed it, but there it was.

However, sometimes even careful reading is not sufficient to prevent difficulties. Over the next few days, using my perspective as both a user and provider of instructions, I'll cover other factors that may interfere with the successful completion of a project.