Schrödinger's cat program
When first run, the program will have a 50/50 chance of doing the following:
A cat program
A standard cat program: read STDIN
and put it on STDOUT
.
Nothing (no-op)
The programs just halt (the cat is dead).
The behaviour is permanent:
Once the program choose between the two options, it should do it no matter how much times you run the script. This can be done by any means but the program should end and remember its choice when restarted (It's fine if it resets when you reboot).