What happened: A plate umpire asked a rookie making his major league debut to straighten his tilted cap, triggering an argument that ended with the manager's ejection. Players on both clubs wore their caps askew the following night in apparent protest.

Why it matters: The umpire later described the standard as an unwritten rule, raising questions about officiating authority. Applying it to caps, socks, pants, shoes and belts would substantially expand crews' role.

What to watch: Watch whether MLB issues guidance before another crew faces a similar cap dispute.