I trapped a wither in a bedrock cage. i then used barriers to block my only tunnel out. the reason was becase i want to be safe in survival but see it too. when i came back a little bit later. they were gone. i replaced them. this happened 3 times, on the 3rd time i used bedrock. no problem, exept i cant se the wither. i heard this was fixed in 14w20a but it is back in. (Note: i didnt ever place the bottom block of bedrock in my tunnel)