I had a part of a setup for a double extender. I expected both pistons to extend.
But instead the pushed piston broke. (picture 2 and 3)
However the horizontal version only breaks if you have a block on the place with the gold block. (picture 1 without block, picture 2 with block)
I am not sure if this is the same bug but it might be https://mojang.atlassian.net/browse/MC-8198
How to reproduce:
1. Build either the horizontal or the vertical version on the picture.
2. Activate the lever.
3. Notice that the pushed piston breaks.