I have a very long commute to/from work every day, so I could ramble on about this subject for hours, but I'll try to give the abbreviated version: nearly every single day, I see people change lanes in the middle of an intersection/change lanes without signaling/change lanes right out in front of traffic moving much faster. I also see people tailgate (often just INCHES away from the person in front of them) even when the person they are tailgating is behind a slower moving vehicle or semi-truck, which makes it impossible for them to go anywhere....but, on the other hand, if you leave any space between you and the car in front of you, people will get all flustered and weave through traffic just to get one car length in front of you...
You also would not believe the amount of people who completely ignore the yellow flashing lights and blow through school zones (specifically on Heatherdowns) at 45 mph. It's really, really horrible.
There's an intersection relatively close to my house in which one street dead-ends into another...there are two lanes and you must either go left, or right...and EVERY day I watch some random driver get into the left-turn lane and turn RIGHT out of it to get around the traffic that has backed up in the right turn lane...
With my long commute for the last 7 years, you'd be amazed at things I've seen people do...I watched a guy get fed up and go around BOTH lanes of traffic using the SHOULDER of the road on US-23...people riding motorcycles down the highway at 80+mph while "popping a wheelie"....I always joke that my commute is going to be what kills me, whether it's from a horrible driver who causes a terrible accident, OR stress from the drive itself.