When a piston with an End Rod tries to push a slime block with another block attached to it, the slime block will break and a slime block will be placed in the world, replacing the original block. Notice that some of the slimes are just pushed weirdly, but in the second picture there are 6 slime blocks (counting the item entity) but I started with just 5.

Rods pushed by a slime block still produce a similar error in Quark 1.1-64, as seen below:

Before:

After: