Place a block 2 blocks above hopper. Walk into the hopper (not creep), when the player does the little jump to get out of the hopper, player starts rapidly jumping up and down and can't exit hopper.
Does not do it every time but easily reproduced. See photo of setup.
Environment
Java 7u65, Win7 64, 8GB, Geforce GTX580, Nvidia 340.52 driver
Confirmed