Some were there on purpose.
While Luke's fighting, Boba Fett flies past him. Or a rag doll looking like Boba.
Like in most Disney film, there's always a nod to characters that aren't actually in the movie. The little mermaid, in this case.
It' dark, but noticeable. There's a sprinkler above this T-Rex's head!
There's some Minion love going on in the back...
The man in the background was either catching a dog in midflight, or disposing of it. None of which should actually be happening.
If this had been real life, Leo would be missing a hand.
In the background there's an ad for 'Batman Vs Superman,' which was released quite a long time after I am Legend. It was in DC's plans, obviously.
This is actually a TV show. Dr. Who fans will notice a Dalek in the background, a nod to the Brithis sci fi show.