I'm not entirely sure that this is not a "feature," but when experimenting with pistons and slime blocks i noticed that in the most recent snapshot 14w18b pistons will receive power from the block they push.

In the past I know this has not been possible. I don't know if it is a deliberate new part of slime blocks but it seems unlikely.

To reproduce:
A.
1. place piston facing up
2. place slime block on top of piston
3. place lever on slime block
4. when pulled piston extends, lever breaks and piston retracts
B.
1. place piston facing up
2. place slime block on top of piston
3. place 2 block on same Y value as piston
4. place a repeater on block directly touching the piston such that if powered it will power the slime block
5. place a lever behind the repeater so that it can be powered
6. pull lever if piston extends what I am talking about is still happening