The huge number of people walking around with headphones/earphones in, who are oblivious to their environment. It's so dangerous. At least leave one ear for listening for traffic etc..
In the past week, twice I've had people walk out in front of me, and once I had to take evasive action because I could see they were going to.
If I hit them, they are likely dead or badly injured, that's bad for them, but I don't want that on my memory/conscience either even if I bear no responsibility for it.