Final answer:
In a volleyball game, a referee might call "side out", "time out", or "out of order", but "be careful" is not a formal referee call.
Step-by-step explanation:
During a volleyball game, a referee might call various things to manage the game and ensure the rules are being followed. One common call is "side out", which occurs when the team that did not serve the ball wins the point, and therefore earns the next serve.
Another call a referee might make is "time out", which is a break in the game requested by a player or coach. The term "out of order" could be used if players do not rotate positions correctly, while "be careful" is not a formal call and is more likely to be informal advice from the referee or a coach.