This happens to me all the time. Perhaps the most amusing instance went as follows (quoted verbatim, with identifying information omitted):

This email is regarding: [].

Class: MATH 7 ADVANCED Prd: 2 Teacher: []

-----------------------------------------------

Good evening, please check [] for missing work, complete it and submit it. Let me know if you have questions or need any help or anything opened up or more tries. Remember the Ch. 3 test due today. Thank you, Mrs. []

I replied:

I think you've got the wrong email address.

Thanks, David

The teacher then replied:

My apologies. You are correct. Your son is crushing it :) and I failed to take him off the group email. Thank you so much for letting me know and keep up the great work! Again, I apologize for the inconvenience. Mrs. []

I then replied:

Thanks! Only one thing: I don't have a son.