Stupidity: Why, oh why, didn't Kirk bring a communicator with him when he beamed down and found himself on the fake Enterprise?
The Enterprise Incident - S3-E2
Stupidity: When the Romulan commander discovers the cloaking device has been stolen, it's clear someone from the Enterprise took it. This is one of the most important pieces of technology the Romulans have ever developed. Rather than take immediate action to retrieve it, she inexplicably allows Spock the right of statement, giving Scotty the time he needs to hook up the device and get away.
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.