If you use a piston to push a slime block, the slime block will try to move all adjacent blocks as if they were psuhed by the piston. If it can't (for instance, if the slime block is on top of a flatworld with no holes), the piston won't push. However, if the slime block is next to a block that can't be pushed by a piston, it won't "try" to push it and will work more or less fine.

I'm not 100% it's a bug, but it's worth reporting anyway.

How to reproduce : build the contraptions in the image. (the left one is basically the control group)