Dispensers are not properly dispensing every time; only some of the time. I discovered this as follows:

I tried to use my fully automatic TNT cannon that works in 1.5 in 1.5.1 and it doesn't work. I have it completely exposed to see all the circuitry, and the dispensers are indeed receiving the correct signal - they are even clicking upon receiving it - but no TNT is coming out (despite my triple, quadruple, and quintuple checks that there is, in fact, TNT in the dispensers). The only thing that seems to get them to dispense TNT is to trigger a block update (neither redstone current nor rearranging the contents of the dispenser will trigger such an update) for the dispenser, and this only works some of the time and then only for a very short period of time (30 seconds to a minute) before the dispenser ceases to work again. Sometimes, one section or another will go once, but not the next time, or the time after that, and then a minute or two later the other section will go once, then the first again, then both, etc, etc, pseudo-randomly ad infinitum.