When I first got the Rocket Launcher, I had a hell of a time shooting down the helicopter that appears right afterwards. Several times I shot all of my rockets (about 8 or so) at it, AND used the Tau laser, and it refused to die.
On the final try, I ended up hiding in the rocket launcher room, and when I poked my head back outside, the helicopter was facing to the side, shooting off into the distance. I proceeded to unload everything I had (rockets and lasers), only to cause the copter to start smoking and spinning in place. I wandered around for a while after that, and eventually found one last rocket I had missed, and FINALLY this one initiated the scripted crash.
Sorry I don’t have screenshots, but I didn’t think of taking any at the time, and have no idea what caused it to turn sideways, as it only did that once.