İ will tryed to make mob trap with doors and piston (not sticky normal piston but i think this problem will not in only normal piston)
Steps to Reproduce
1. Make mob trap with piston
2. Take creeper spawn egg
3. spawn creeper under the piston
4. Push piston
5. Creeper doesn't die! But piston still pushed!
Creepers never die with pistons
Pistons and Piston Arms are transparent blocks, which do not suffocate mobs.