See the screenshots for the obvious examples.
When you have two sticky pistons on top of each other, they are both retracting blocks and the top one is a honey or a slime block, then upon retraction, the top block (slime/honey) will remain behind.

//EDIT

It is because the bottom piston is bud-powered...
So in steps
You unpower the lever, causing the top piston to retract.
The piston can't grab onto the slime block, because it is stuck to the yellow concrete block, so the slime block remains behind.
Because of the retracting top piston, the bottom one gets updated and also fires, successfully retracting the yellow concrete.

Tada, explanation found.