Last summer on an Edinburgh to Gatwick flight, I went into full on stern teacher mode over a person reclining their child's seat.
A woman with an infant was sat in the window seat next to mine, due to the confined space and the fact that her baby had on some type of metal brace contraption attached to his legs, plus he was wearing a helmet, getting the baby extender seat belt on him was difficult, so of course I helped her get settled.
In front of us was a woman sat on the aisle seat with her two small children in the two inside seats. The window seat child was blasting his video game to the entire plane. It was so bad that the flight attendant asked him to mute it or use headphones. His mother grumbling pulled out headphones for him to use. He used them for about 10 minutes and then went back to blasting the audio to the whole plane.
When she wasn't dealing with her child, she was staring at the baby next to me, who by the way was super well behaved.
As soon as the plane hit cruising altitude, she promptly reclined her son's seat so that it was crushing the infant next to me. I immediately tapped her on the shoulder, gave her my best teacher glare and told her to put the seat back as it was crushing the baby. She apologized profusely said she hadn't seen the baby and put her son's seat back into it's upright position.
The mother next to me thanked me.
While I understand that people have paid for the right to recline their seats, it is common sense that you should never crush a baby!