Droppers/Dispensers fire irregularly (just some of them) when placed next to each-other and powered from a redstone wire on top of them (reproduce as in the attached figure: line of blocks powered by redstone dust and array of droppers with random items below).

It seems that they remain budded before the wire current is stopped (if after one button press the droppers are manually updated, they will all fire at the next press).

I have tested in Minecraft 1.7 as well and this behavior is not there, they all fire as expected.