I just opened a pack of props I ordered from Apple, and found that one of them was very hard to put on. It's like the hole wasn't big enough, or something wasn't aligning right. I remember seeing someone on this forum who had just received their drone who said they had a hard time putting the props on and one flew off causing a crash right away. I think I see what happened there - they had a bad prop like the one I've got here, and because they had just received the Mavic they didn't know how easy it should be to attach a prop, so they tried to fly with it. Obviously I'm not going to attempt to fly with my bad prop, but this should be a warning to new people - if your prop doesn't go on easily (and you know you're putting the right one on marked/unmarked) then don't try to use it.