Two from a recent flight:
When someone wants to get by you into your seat row, hold your iced coffee in your hand while standing and waiting. Do not put it in the seatback pocket to store it, the person will bump it and it will collapse and spill all over your stuff under the seat. (note that this happened before caffeine was in my system, hence the resulting poor choice).
At cruise altitude, the air pressure in the cabin is less than on the ground. Normally this does not present a problem. However, if your child has recently acquired a water bottle that uses a straw to draw the water out, you will get a quick lesson in fluid dynamics when you go to open the straw for the child at altitude - water shot right up the straw in a pretty impressive fountain, hitting all three of us in the row before I was able to close the straw. Equalize the pressure by unscrewing the lid first!