I know with "speak of the devil" there used to be a lot of suspicions that saying the name of a demon/malevolent entity/the devil will tend to wake/summon them. So when someone appears just after you've mentioned them -> they're the devil!
Push the envelope I always assumed (don't know this one) was about the 'envelope' of sound surrounding a vehicle, so if you get towards the leading edge (ie get near to the speed of sound) you're 'pushing the envelope' - just before you break through.
I always wondered about cats and dogs - letting the cat out of the bag, and why the dog's b******s means something good!