Weaving at high speed in relatively slow, relatively heavy traffic approaching a series of lights. This 20-something in daddy's beemer accelerates rapidly up to a red light, cutting off my own lane change, which I signaled well in advance and gave plenty of space to the car that was in the lane beside me. Beemer was at least 4 car lengths behind me when I signaled and began my lane change, and then he rapidly accelerated, fording himself beside me before I was more than a few feet into the lane. To do so, he had to accelerate rapidly, coming up to a red light and then coming to a fairly abrupt stop. (The kid also made a few crude gestures and shouted some profanity. I offered some advice about maintaining an appropriate speed, without not weaving or cutting people off.) He did not need to be in that lane, it was just weaving. (I did, which is why I made space enough for a safe lane change before his unsafe maneuver.) He switched back almost immediately, continuing his weaving, tailgating, etc. after the red.