Creating a slime bridge creates artifact blocks.

When creating a retractable bridge using sticky piston and slime, upon activating the retraction or extension of the bridge, there is a chance that phantom blocks (either slime or the block attached to the slime) are created. These phantom blocks can be stepped on; however, breaking the block does not yield anything and occasionally is destroyed when the block is moved again. I found that slime would disappear when the mechanism returned and reconnected with the phantom slime; however, the attached block did not, and was pushed by the mechanism.