I was slightly distracted while driving, earlier, and pulled into the wrong lane at some lights for the direction I wanted to go. I flipped on the indicator, checked the surrounding traffic, waited for the lights to change. As they did, I started to pull left, only to hear a horn behind me, and see a motorbike braking hard in my left-side blind-spot. (Right hand drive car.) He'd been filtering between the lanes a bit quickly, and I hadn't seen him at all. He could have been riding more defensively, but it was still wholly my mistake.
It was a nice day, so I had the windows all the way down. Motorbiker came up to the window and I said 'I'm so sorry, that was my fault'. He started shouting and raging for a second as if I'd just blamed him, then did a literal double-take and said 'what did you just say?' I apologised again, he looked nonplussed, apologised as well, and then rode off.