Discovered a very specific "feature" troubleshooting my parent's TV setup.
One HDMI input is hooked to a DirecTV receiver, and one set of RCA inputs is hooked to a old ass DVD player. The DVD player has a manual power button; switch off puts it into standby mode, switch on energizes the player. There is no on/off via remote, just playback controls.
With the switch off DirecTV works, but when you hit the switch the HDCP in the DirecTV receiver kicks on and displays a "TV does not support this program's content protection" error message. Apparently, the DirecTV receiver can detect when the RCA input on the television receives signal.
Fascinating.