I used to commute by bike until physical problems interfered. The biggest infractions that I've observed are (1) riding, often at speeds exceeding the pace of pedestrians around them, on the sidewalk, (2) riding against traffic - I almost hit one as I was exiting a parking lot because you don't expect someone to be coming from that direction, and (3) not observing traffic signals and signage, e.g., riding without stopping from the two-way section onto the one-way (in the wrong direction) section of Essex Street at the intersection with Dummer Street - again that's not the legal flow of traffic.