Description:

As the title says, upwards facing pistons - both normal and sticky - won't activate one block below the build limit.

I think this is definitely a bug, since pistons should still fit under the build limit when they're one block below it and extended.

Steps to reproduce:

1) Place a piston on level 255 - standing on (254 - looking at).
2) Place redstone block next to the piston.

Below is a screenshot of the bug:

EDIT:

This bug also occurs at the bottom of the world when the piston is placed facing downwards on the second layer of the world.
For survival, with default world generation, this isn't a problem, but for creative worlds, it certainly can be.