Chromebooks and Chrome OS have had a relatively smooth existence since they were first released way back in 2011, but some users are now starting to report a pretty inconvenient bug. The glitch is tied to the external monitor features built into the devices and a wide array of devices have been affected on the "stable" software channel. It has been reported to occur on the Acer R11, as well as the Acer 14 Chromebook, and some unnamed Lenovo models. Unfortunately, there is no real way to pin down exactly which devices will experience the problem as of this writing so it is something for users to be aware of.
Ordinarily, bugs are to be expected on any of the more "developer-focused" software channels available to Chromebook users through settings. However, Chrome OS sees updates with high frequency and there is no such thing as perfect software, so bugs are possible on the stable channel from time to time. Google even offers rewards every year to those that can find and replicate any more serious bugs in both Chrome OS and Chrome, as a way to help minimize issues. Luckily, this most recent bug doesn't appear to be a big deal. That said, if you've been using your Chromebook's HDMI port to push movies, music, games, or just your browser to a bigger screen such as a television, this new issue could really bother you. Users are reporting instances where, after just minutes of having connected to a 1080p screen via HDMI and closing the device, the display screen goes black. The Chromebook itself immediately becomes completely unresponsive once that happens. The only way to recover from the blackout is to completely reboot the device, presumably by holding down the power button until it shuts off. Fortunately, the bug reportedly only occurs when connected to a 1080p screen and after closing the device, which could save some users from having to deal with it.
Since users already reporting the bug across several Chromebook devices, developers should be able to narrow down the problem fairly quickly. Once the origin of the problem is discovered, a fix will most likely be pushed out in short order because the stable channel is the channel most used by everyday consumers. If you are experiencing the aptly-dubbed "screen blackout bug," the best course of action is to report the bug and wait for a fix or workaround to appear.