I don't really feel like calling this a bug, because it seems more like a programming miscalculation. This is a bug where when any piston (sticky or normal) is extended, there is a very small area that you can walk through, aka no hit box present.

Duplication:

1. Go into any minecraft world (conditions don't seem to matter)
2. Have a piston extend WEST (other directions don't seem to do this.)
3. Within the block of the actual extended part, Line up your X coordinate to somewhere within .05 and .075 relative to that block's position, and now you can walk through it.