…someone who is constantly trying to act in a position of authority, even though he has none? Telling people that they’re doing whatever they’re doing the wrong way, like ringing a stranger’s doorbell to tell them their flag is backwards, or stopping strangers in a parking lot to tell them that they shouldn’t have their child safety seat in the front seat of the car? I’m thinking “raconteur” or “factotum”, or simply “buttinski” or “noodge”, but I’m told none of those are quite right.
I’d simply call them a busybody.
Busybody (I see XJETGIRLX thinks so too). Or: know-it-all…meddler…Try the thesaurus…
GT
Years ago I heard someone use pansophist to mean precisely what you are talking about.
[Lucy Van Pelt]THAT’S IT!!![/Schroeder goes flipping through the air] Thank you!
I thought they were called junior mods… 
Dwight Schrute.
It can be used as a verb as well, e.g.:
“Man, I hate going to work. As if it wasn’t bad enough that I have 4 bosses watching over me, half of my co-workers are constantly Dwight Schruting me when the managers aren’t around.”