Since 15w38 items, that are squeezed against other blocks, receive a lot of momentum. This causes them, for example, to fly out of the top of 3x3 glass elevators.

One harmful side effect of this change is that saplings in automated tree farms, where pistons break the leaf blocks, are spread in a much larger area. The few hoppers, that were enough to collect saplings in 1.8, would only pick up a fragment in the versions since 15w38. The majority of the saplings now fly through the back of the pistons and also seem to glitch through blocks and land in the redstone wiring. This more or less breaks the concept of automated tree farms.

Maybe it's possible to give the items less momentum again like in 1.8 or at least not so much. The increased speed is mostly an annoyance.

How to recreate to visualize:
Build an upwards facing dropper and several layers of 3x3 full blocks centered on the dropper on top. Depending on the version the items fly with more or less speed through the full blocks on top after activation of the dropper (see first picture)

The other pictures show how far the saplings are propelled outside of the tree farm chamber.

Code analysis by marcono1234 can be found in this comment.