If a TNT explode while a piston is moving, the "piston_extension" can be destroyed and leave the "piston_base".
I think you can set the resistance of "piston_extension" to 6000000 to prevent it.
Environment
System.getProperty('os.name') == 'Mac OS X'
System.getProperty('os.version') == '10.11.6'
System.getProperty('os.arch') == 'x86_64'
System.getProperty('java.version') == '1.8.0_74'
System.getProperty('java.vendor') == 'Oracle Corporation'
System.getProperty('sun.arch.data.model') == '64'
Dupe of MC-27056