When two or more people touch each other inappropriately, most of the time it's by accident. from here the graze can take two paths 1) all parties involved know that a graze has taken place but choose not to say anything because it may lead to awkwardness. 2) one or more of the parties involved informs everyone present, involved or not, that the graze has taken place...awkwardness follows.
Guy 1 brushes his hand (accidentally we assume) against guy 2's ass
Guy 2: "dude, WTF?"
Guy 1: "Accident?"
*a graze has just occured