Nest CEO Tony Fadell announced today that it would stop selling its Protect fire alarm while it reconsidered a key feature of the device: The ability to "wave to hush" a warning alarm. He explains that the issue is with the alarm's motion detection system.

The system is supposed to be able to silence a warning with the wave of your hand but further tests have revealed that the feature could be triggered by accident during a real fire. There's danger to this because, it could result in a delayed response to actual danger. From the company:
We discovered that movements near the product that are not intended as a wave can be misinterpreted by the Nest Wave algorithm. If this occurs during a fire, this could delay the alarm going off. So, we are disabling this feature until we have a proper solution.

This is an issue we observed while performing laboratory tests. To the best of our knowledge, no customer has experienced this issue.
If you already have a Protect, the wave feature will be automatically disabled within the next day. You will need to connect your alarm to your Wi-Fi in order to get the updated version of Nest's software.

The company isn't changing the hardware, just the software, so for now, even if you have a Protect, all you need to do is wait for the software update. More information is on a Q&A about the issue here.