All dynamic entities seem to be invisible. Zombies still aggro, make response noise and attack and cause damage. However, they are invisible.

Item spawns appear to be non-existent or are also invisible.

This began approximately 10:30pmEST, or at least that was the soonest that I realized it. I tried several other servers only to notice the same behavior. I quit and restarted the client, only to continue to see the same behavior.