Along with people trying to light enormous spaces with tiny on-camera flashes, a photographic peeve of mine is when I’m working as a paid or official photographer, have put together a large group shot, and then one or more people sneak up behind to try to take it themselves with their cell phone.
The inevitable result is that a good fraction of the people will be looking at them, not me, thus giving the photo a confused and jumbled look because of how sensitive we are to the direction in which people’s eyes are pointing.
You can see some evidence of what I mean in this shot, though I put in a significant effort to draw people’s attention to my lens and shoo away the amateurs (who are always so wounded and pained about being denied, perhaps especially after being told why there is a good reason for it). Here is one where I made sure it did not happen, and you can see the unity of gazes which is generally desirable in a shot of a large group.
If you want a copy of a group shot being done by a professional, please wait until they are done and politely ask where it will be posted or if they will send it to you. That way, you won’t spoil the official record through the process of creating your own low resolution cell phone derivative which nobody will see.