When pushing a string of blocks with slime blocks, the retraction of a block sticking out will not move entities.
For example: An "L" shape of horizontal slime blocks where one end is attached to a sticky piston. When powered, the blocks push out as normal and push anything in their way. However, on the way back, if there is a mob/entity on the back side of the block(where to block will go once unpowered) the block will continue straight through the entity without pushing them. Any push afterwards is the result of the entity being inside the block. From my tests, the same applies in a vertical system as well.
Steps to Recreate:
1. Create a piston with slime blocks that attach perpendicular to the axis of extension.
2. Power piston and have system extend.
3. Place mob or entity on backside of blocks.
4. Unpower piston.
5. Observe that retraction of blocks does not move entity.