When you place a fence on top of an activated piston and you tie a mob to the fence, the lead has a delay in breaking when you deactivate the piston. Note that doing the same thing but with the piston initially being deactivated and then activating it breaks the lead instantly, as intended.
Environment
mcbug1 is when i first discovered it, and mcbug2 was when i tested it on a superflat world after finding out.
This is being tracked in MC-13780