As shown in the picture, a piston pointed sideways will jam in extended position if:

-its powered from a block above thats powered itself by redstone lying on top of the block AND

-if another piston pointing downwards on the block piston 1 is pushing at AND

-if piston 2 is powered by the exact same block as piston 1

I tried it in many other positions and only this single one is causing the jam

[Edit] trivial but still: the empty wool block in the picture is where i powered