For a while after updating to El Capitan, I've seen those weird marks in front of every line. I thought it was an incompatibily with oh-my-zsh. Turns out it isn't and I've discovered that through John Siracusa.

He links to a wonderful stack exchange answer. It explains in details what they are and how to change their behaviour, so I won't repeat it here. I just want to highlight the keyboard shortcuts to navigate through them:

• Cmd+Up/Down lets you move up and down between those marks.
• Cmd+Shift+Up/Down lets you select the text between marks.

I found this very convenient to copy/paste the output of a script. Will use a lot.