Put the summary of the bug you're having here

*What I expected to happen was : I thought they would not extend
*What actually happened was: Spread and remained so

Steps to reproduce:
1.Put a block with a redstone torch above
2.Then put line of blocks with redstone connected to the torch
3.Put a piston below the line with redstone leaving air between these
4.Place a block above the pistons and destroy it and the line with redstone