If you place redstone comparators pointing into Dispensers and give the dispenser (or dropper) a constant input it will cause it to rapid fire items.
See image for a setup example to test this bug.
MC-7649
Rapid pulse bug with dispensers & droppers
Resolved
Duplicate
0
Chris Giles
(Unassigned)
Unconfirmed
Snapshot 13w02b, Snapshot 13w03a
Retrieved
Duplicate of MC-846 and MC-6077, please use the search function to see if your bug has already been submitted. Currently over 45% of tickets are being closed as duplicate.