When a Dropper or Dispenser block is powered, it becomes sensitive to redstone updates and will dispense an item if an update occurs nearby (e.g. unpowered redstone being placed next to it, or the redstone powering it changes power levels). In this way, a powered Dropper/Dispenser can be made to fire repeatedly by placing an unconnected clock nearby (tested with unconnected redstone 1 block away).
I suspect this, when combined with the (now fixed) Comparator bug that cause it to constantly provide redstone updates, is why a powered Comparator pointing at a Dispenser or Dropper could cause it to rapidly fire.
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Sorry about that. I actually did use search before I posted as it seemed like a pretty easy-to-discover bug, but must have missed that one.
Duplicate of MC-846, please use the search function to see if your bug has already been submitted. Currently over 50% of tickets are being closed as duplicate.