seaweed is nasty.
having your kite lines hook up on someone else's kite while they are both flying is also bad.
Stunt kites wrapped around your lines dont to great things for your kite's flying ability either.
boat mooring bouys are interesting when you catch them trying to re-launch
Most random thing I have had happen was a branch falling and tangling in my lines once when we were kiting just downwind of a bunch of low trees.
Best one I have heard of is a dude who launched with a board hooked up n his lines, and when he brought the kite to 12 oclock the board slid down the lines and hit him in the head!