During a Rocket League rumble match, a frozen ball began falling without any car touching it, and without unfreezing. I know from personal experience that nearby demolitions and plungers can unfreeze balls, but there were no plungers or demolitions at the time.
An earlier answer suggested that it was because the ball hit the wall while it was in its freezing animation, but from my own testing, it looks like the ball still freezes properly in that situation:
What could have caused this frozen ball to drop while still being frozen?