A common style of "Portcullis gate" uses gravity blocks like sand or gravel to push up fence posts up by at least two layers. When the pistons retract, both layers should "resettle" as solid blocks as shown in the following video:
On Pre6, one of the layers "breaks" so there is now only a single layer, so the fence posts can only be pushed up by one layer and the gravity block items will despawn.
It looks like something changed in the falling entity timings such that one entity falling upon another becomes "broken" faster than as happened with prior versions. This will break all "falling sand" portcullises working in prior versions, and a work-around becomes a huge problem with double-retracting pistons to pull the "gate" back down.