Every person who has ever taught a class has made an error of some sort. Sometimes you know you’ve made one immediately, while sometimes the mistake becomes evident only after a period of time.
I was recently contacted by an adjunct professor who had been provided by her university with a syllabus, lesson plans, assignment instructions, grading rubrics, etc. for a very technical class. One of the assignments had a set of relatively complex instructions. Five weeks into the project, she discovered to her horror that she had interpreted the instructions for the assignment in a certain way, and while half of the class had followed her lead, the other half of the class had interpreted them completely differently. Because of this, this second group was generating work that was different than that of their classmates. How should a professor handle something like this and still be fair to all?
While there were a number of solutions to this specific problem, the key to resolving any problem that arises from a teacher error is to admit to the class that you made a mistake. Give them a brief outline of what happened- they don’t need to know all the details, just enough to give them a picture of what led to the confusion/misunderstanding/error. This shows them that you are on top of it now, and that you understand the necessity of working towards a fair solution. Then present them with that solution, which is just what this professor did.
In this case, the best solution had to strike a balance between what was done by both groups prior to discovery of the error, what the assignment originally intended (learning outcomes), how well the work already done by both groups would achieve the learning outcomes of the assignment, and importantly how the professor wanted to grade the assignment from this point forward.