In previous versions of Minecraft, repaters could send redstone signals through blocks by just giving a redstone input.

In the 16w42a snapshot a redstone signal only passes through the block when the redstone on top and behind the block recieves a block update. When the signal gets turned off again, the redstone on top and behind the block will stay activated and only turn off if it gets an other block update.

Steps to Reproduce:
1. Place a lever
2. Right next to the lever a block
3. Put redstone on top and behind the block
4. Flick the lever to trun redstone on (Redstone on top and behind the block won't recieve a redstone signal)
5. Block update the redstone which didn't turn on (by placing a block or something next to it, redstone will now turn on)
6. Trun lever back off (Redstone on top and behind the block will stay activated)
7. Block update the redstone again (by removing the block again, redstone will turn off now)