Be cautious when passing stopped or slow-moving vehicles. They often hide dangers that are not immediately apparent. The first thing to ask yourself is why? Why is the vehicle moving slowly, or why is it stopped? This rule is about patience, courtesy and taking care for others.
Stopped vehicles may have people around them who are preoccupied -- perhaps changing a tire, working under the hood, or even working under the vehicle with their legs stuck out into the road. It's not uncommon for someone stranded beside the road to forget where they are and be struck and killed by a passing vehicle when they inadvertently step into the roadway. If you see a vehicle stopped along the roadway, give them extra room. Change lanes if you can. If you cannot, at least move left in the roadway so if someone does get in the way, they will be easier to miss.
In the same way, if you come up behind a vehicle stopped in the roadway (instead of beside it), be wary. Instead of automatically going around them, slow down (or stop), and figure out why they're stopped. Maybe an animal or a child is crossing in front of them. This is also a concern at a crosswalk; it is often against the law to pass a car going your way that is stopped at a crosswalk (even if you are in an adjacent lane), because that vehicle may be stopped for a reason that you cannot immediately see! Don't pass until you are SURE it is safe.
Use caution when approaching a slow-moving vehicle. It creates danger when vehicles are moving at different speeds in close proximity to each other. If you are flying up behind another vehicle, the risk of a collision is increased if a conflict develops, plus another driver moving at a very slow speed should cause you to think that something might be wrong, so you could expect other erratic behavior as well. Be prepared to react and slow down until you see that it is safe to go around.
Exercise care if you are coming up behind a line of slower vehicles. It is an error to take an immediate opportunity (if presented) to pass the entire line. Some of those drivers may have been in that line for awhile and they may be getting impatient. They may "bop" out in front of you, so plan for that possibility. Again, slow down. If you are determined to pass, do so safely by passing them one at a time -- and let them know you are coming around by a quick "toot" on your horn or flashing your headlights quickly. It increases your safety if they know your intentions. This same rule applies if the slow vehicles are in a lane beside yours -- you should remain alert to the possibility that any one of them might suddenly change into your lane, and be prepared to avoid a collision if they do.
Keep it between the fenceposts!