Fairly certain it wasn't even remotely illegal before 9/11/2001, after which it probably did become pretty illegal (for the opening of the cockpit door).
It'd be pretty hard to ever prove anything, it's not like they can really go hundreds of miles off the flight path. In my case, the cockpit door opened completely on accident, or at least because of what the pilots were doing, the turbulence is what bumped the door open. Now that the doors are locked, it wouldn't happen again. They blushed and shut it as quickly as they could, and later offered an apology that didn't openly admit to doing it intentionally, but reading between the lines was as close as they could go.
It wasn't particularly enjoyable at that moment, but it is really funny, and I'm glad it happened.
