People on their phones.
In cars you really only see the extent if you're paying attention of the speed of the vehicle or notice them veering around from behind but it's surprising how much you notice on a bike. I particularly like filtering through traffic and looking through the driver's window at the guilty party as they sit there with the "I've been caught" face..... I must pass 3 or 4 on every commute.
The worst offenders are van drivers and women in 4x4s sat in near-stationary traffic, texting. Waiting at lights also tends to be a hot spot for it but the most dangerous; people glancing down at their handset at 70mph+, thinking they're getting away with it. The amount of times I've seen someone glance down, look up and slam on the brakes because traffic has stopped![]()
On a similar note.. People actually on the phone, i.e. holding it to their ear and having a conversation with someone, in a modern car that obviously has bluetooth functionality. Are these people too dumb to configure it? Can you even buy a car without bluetooth nowadays?

). I could be wrong I just thought the highway code said Vasily first come first serve irrespective of which side the obstacle is on?
.