When using a sticky piston setup in which there is a retractable extender (two sticky pistons pushing/pulling a sticky piston and a slime block, see image attached) in some random cases the sticky piston will not retract, and will remain that way for that coordinate pair permanently. The images below show the bug in detail. What I have done here is set up 8 basic retractable extenders. I set a redstone torch in each pit, and a redstone torch activating the two back pistons, then removed them in order to reset the pistons back to their original setup. In 5 out of the 8 cases, it worked fine, but in 3 cases, the pistons did not correctly retract the slime block. this has been repeated on multiple worlds, and the issue stays on those 3 setups no matter how many trials are completed, while the other piston setups continue to work correctly.

Edit for newer version In all the latest releases, the slime does not pull back at all. Every single piston set up does not work correctly.

Another setup is shown in MC-85581.