Dispensers fire when updated by non-redstone updates

Unlike the usual dispenser behavior where dispensers fire when powered and given a REDSTONE update, dispensers fire when given any block update (while powered).

Demonstrated here: http://www.youtube.com/watch?v=qyzFkmp3G3c&feature=youtu.be

As seen above – Lava gives a block update to dispenser, causing an infinite loop of updating and firing to occur.