When teachers compare you to your siblings, even though you both achieved the same result!
^ This reminds me of a particular incident in grade 11 (final year of O levels). My Econ teacher hated my guts. She was a 45-year old unmarried, semi-crazy lady who used to teach O level Economics from an A level textbook (no joke. Had we practised A level past papers, we'd have written final A level exams!!).
For some unfathomable reason, she had a major grudge against me and used to constantly compare me with my sister (sis was better in the subject, but her classwork/participation/grades were identical to mine). But she would leave no opportunity to pick at me and ridicule me in class.
Final year, in fact 2 months before the exam, she asks us to go and buy a BA book, "Economics of Pakistan" worth around 55 rupees iirc. Why? "I want you all to memorise all the facts from this book and sprinkle your answers with facts from the Pakistani economy." All this, when we were clearly over-prepared for exams ANYWAY. I personally snapped. I didn't buy the book. In a classic triple-period Econ session, I set her straight. I sat at the front seat, hardly a foot away from the teacher. No "Economics of pakistan" on me.
Teacher: "BL, where is your book?"
Me: "I didn't buy it."
T: "Why? Don't you know you need it? Why do you always do this? Did you know, your sister never---"
Me: "4 years ago, she listened to you and bought the book, yes. It's still in our cupboard, rotting somewhere. I don't need it. I've a sheet of paper on me, I'll take down any points I feel are necessary."
T: "You will not, you will share the book with somebody."
Me: "No thanks."
T: "Share it."
Me: "No."
T: "I said share it!"
(By this time, my two best friends, sitting beside me, began to share their books with me. I pushed them away. They broke into a series of giggles)
Me: "No." (cool as ice, gaze levelled with hers).
She looked at me for 10 seconds, fuming. I smirked at her like no one's business. Around 5 students burst into laughter at the scene, and she started scolding them/threatening to report them. She drank from her bottle of water, then ordered one of my best friend's to read (who was having a hard time due to her constant giggling).
She stormed out of the class half an hour early

This incident became pretty popular amongst all the Economic students of that batch