I was trying to make a anvil trap that looks like a piston door and got it all set up but instead of cobble stone that i was going to use,i decided to use obsidian instead to be able to get out if you have a diamond pickaxe but when i tried to push the obsidian with the pistons i would not work for me.

To Recreate
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1. start by making a one way piston door and get it set up for action.
2. instead of any other block use obsidian instead.
3. activate the piston and it will not move.
If you remove the obsidian it works fine though.

EDIT-
I tried the same with bedrock and that won't work either.