Driver (often of van or other large vehicle like a bus) flashes you into the traffic in front of them but only slows down enough to let you in if you spot their signal instantly and move like greased lightning.
Then when they have to brake a bit more because you haven't instantly sped into the space, they get all huffy.
For some reason this gets me very annoyed. Especially as sometimes they're still going so fast that I'm not convinced they're letting me in, so I hesitate, and then they flash and flash and flash and glare.
Bah.