This should help you. http://bfy.tw/O4wM
I was able to replicate this issue too. I also tested to see if the live coral fan would be moved by the piston, but it wasn't. Once it turned to its dead state, it was able to be moved.
Confirmed as well. I had a mob farm working fine in pre7, but pre8 broke it due to this problem.
Wow, Mojang. Any idea when you're going to get around to fixing this THREE YEAR OLD ISSUE!!!!
First of all, I disagree with this being a duplicate of MC-11193. It may have similar issue, but the issue I experience in 1.11 DID NOT HAPPEN in v1.10. It worked fine in v1.10. It broke as soon as 1.11 was released. The MC-11193 you say is a duplicate was reported back in 2013. Apparently, NOTHING is being done about it. FIX IT!!!
I didn't see this issue before submitting mine, but that's because I was looking for the keyword villager in the subject and should have searched for farmland or similar. Anyways, on mine, I attached a video showing a villager falling through the block. See MC-104906. Hope it helps get it resolved soon.
Not a bug. If your game difficulty setting is on normal or hard, they inject venom in you. See Wiki under Venom section: https://minecraft.gamepedia.com/Cave_Spider#Venom
Are you sure you weren't playing on easy mode? Also, don't get them confused with regular spiders.