When a dispenser is powered, and contains such items like buckets and flint&steel, it continually uses it while the signal is on. Does not happen with arrows, potions or blocks. I haven't done complete testing, but it seem like it only affect buckets and flint&steel.
At first I noticed this bug while in creative, wanting to use dispensers to light things up. While receiving one signal, it continually uses the item. The same with buckets, there's no way to predict whether the water will be picked up or not as it rapidly pulses.