I had a one tick clock going and the comparators weren't allowing a pulse to go through (which could be a bug itself), but when I repeatedly right clicked the comparator, I was able to get the pulse through as you can see in the screenshot.
Code analysis by @unknown can be found in this comment.
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This is a shot of a test device, a very simple redstone clock using a comparator in subtract mode feeding itself.
This second screenshot shows the piston responding when I right-click the second comparator, though it usually takes a few clicks to get it to happen. It pulses really quickly so this is the best shot I got, but as you can see it did allow the signal through in 13w10a.
Okay so it's still happening all the way in 15w42a, and here's a gfy of the setup and results.
I was able to semi-confirm this on 13w07a, in that if a comparator is being fed a one-tick rapid pulse, it will not allow it through until it gets updated. Some designs update the comparator and so allow the signal through, while others do not and the comparator needs a block/redstone update to start letting the signal out.
Ones that worked had the comparator near other equipment that would update at the time, non-working designs (ones that needed an update) tended to have the comparator a few blocks away from the equipment sending the signal.