Slime blocks extended by pistons don't launch players upward if there's a block stuck to it in the direction of the player's approach. Examples include the Super Mario mash-up and the Toy Story mash-up, but it seems to happen in every world. This only occurs if the block gets stuck to the piston, glazed terracotta produces the expected behavior.

Steps to reproduce

1. Place a sticky piston with a pressure plate and a 4-tick repeater running into it and a slime block on top. (You could probably place it any direction, but it's easiest to notice going upwards.)
2. Make sure the block between the pressure plate and the slime block will stick to slime blocks.
3. Run at the piston and trigger the pressure plate

 Expected Results

The player gets launched up by the slime block.

Observed Results

The player appears to be getting stuck on the block next to the slime block, preventing them from being launched.