1. A Control of Error
A control of error gives a child a way to self-correct the work they have completed. One example involving the sound cylinders includes matching colored dots on the bottom. Once each pair has been matched, the child can turn the pair over to ensure the match is correct. This allows them to correct their work without correction from an instructor. Another example is the cylinder blocks (also known a knobbed cylinders). When a child successfully completes the works, every cylinder has been placed in a corresponding hole. A final, excellent example is the spindle box. When the spindles have been correctly placed in the spindle box, none will remain. If the child does not have enough or completes the activity with remaining spindles, the child can clearly see that the activity has not been completed correctly.
2. A Point of Interest
A "point of interest" is a bit more abstract. This is any detail that will spark the interest of a child. Maria Montessori believed that works should contain various points of interest to prevent frustration. Examples are often sensorial in nature. For example, children are often intrigued by unusually shaped bowel (visual) or the sound that items make when being transferred (auditory). My Montessori Journey has a wonderful post on identifying points of interest.
3. A Point or Isolation of Difficulty
One of the key points of a Montessori work is the isolation of the concept being taught. This is extremely important and can often be harder than it seems. One example is how the color tablets are introduced. The first set presents the primary colors red, yellow and blue. The tablets are identical in every way except for color. If I were to present a banana, blueberries and apple, the focus would instead be on the variations of the fruit in addition to the color. That would be enough to confuse the child regarding the ultimate goal of the exercise.
Hopefully, this will help you differentiation some of the critical aspects of creating a Montessori work.