An un-active dispenser trap keeps shooting me when I am facing away from it.

I expected my trap not to fire until it received a redstone signal from the tripwire.

As I walked away, it shot me in the back.

Steps to Reproduce:
1. Set up two dispensers (one on top of the other and both facing the same direction) and then, with a five block gap, place another two dispensers facing the first two.

2. Place a tripwire hook on each of the bottom dispensers (facing the opposite two dispensers) and place one string in front of the tripwire hooks while leaving the middle place without string.
3. Place a half slab one block above and to the left or right (so that it looks as if it is... correct to gravity?) then place a redstone lamp over the slab and have a lever on the lamp, leaving a one block gap between the lever and the top dispenser, activate the lever, stand in front of the two dispensers (closest to the lever), and walk straight to the opposite two dispenser... (and get shot in the back!)
4. If this doesn't work the first time then try looking inside the dispenser, turning the lamp off for ten seconds, turn it back on and then try again.

Ps. I had to try to recreate the bug myself because I couldn't believe that I had been shot by a trap that shouldn't work! Also it will not shoot you if you are looking at the dispenser (the one closest to the lever) and don't block with a sword. I find that it works most times if your looking down. Don't sprint either.