Tailgating in traffic jams, allowing no room for merging or changing lanes, causing everyone to have to slam on their brakes when someone does need to move lanes.
You're overestimating the frequency of necessary lane changes. You either move left to pass or right to exit. In heavy freeway traffic there ain't a lot of either going on. Too many people changing lanes trying to chase the faster moving lane causes a lot more problems.
Waiting until the absolute last second to merge when lanes are reduced.
I saw this often enough and what I noticed is that these people would often fill in spaces that idiots opened up by failing to pay attention and
stay exactly right behind the car in front.
I'm not sure what you're advocating here. Everyone maintaining a fixed distance behind the car in front creates a rigid body effect. I've also noticed less last minute merges going on at the 405 and 101 interchanges in LA as more people pay attention, leaving fewer gaps to fill which discourages the number of last minute attempts.