Was riding on minecart towards a farm when another farm loaded into view. The farm uses a redstone clock system with pistons which activate and deactivate redstone elements. Though the entities were loaded, tthey were visibly obscured by the structure of the farm. While obscured, one of the pistons must have become active. It is likely at this moment when the game crashed; stating it did not have entity data for sticky pistons facing south. [Edit] After reviewing the issue again, it seems that the crash only occurs if the player is looking at the pistons when they activate. If they keep the pistons out of view when they activate the game does not crash.
Since this seemed like an unusual issue, in a new world I placed pistons in all positions (except facing down) and orientations to test single instances of moving pistons. I could not reproduce the issue with an suite of pistons.
I am unsure therefore what the actual end issue is, the bug report has been included.
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Please attach the affected world to this report.

Zipped archive or .dat? The archived version is ~180MB in size, far greater than the file limit.

Zip would be fine. If it's too large to attach here directly, you might need to use a file sharing service of some kind to link it here.

May the crash be related to the activation of pistons?

I hope it is fine for me to also comment here. I got the same problem and can also track the problem to the hopper clock, more precisely to the pistons it is working with. The bug is existing in MC 1.10.1 whereas everything is working fine in 1.10. I even copied the world and restarted it in 1.10 to confirm this.
http://www.file-upload.net/download-11700464/NewWorld-Kopie.rar.html
My world as zip file so you can have a look if needed.

It has been recently identified that pistons can cause this crash. Currently we only need a world save where it occurs without pistons (if this does happen). Thank you however for the attachment.

Dang, it took so long for my world to upload that I just now read your comment. Regardless, my world file can be accessed at: https://drive.google.com/open?id=0BxEnoZ8uuBDmbkFjUi1zMU5rSUU . If I encounter the issue wherein the crash occurs without pistons I will be sure to attach it.

Thank you. It will be helpful should you encounter it.