On Reddit, the big news of the day was that some Android handsets running Pie or a Pie beta (like Pixel models, the Essential Phone, Nokia 7 Plus, and the OnePlus 6) were turning on the Batter Saver mode even if the battery on the phone was almost fully charged. For those unfamiliar with this feature, it is supposed to keep a battery running for an extended time by preventing apps from running in the background. It also stops apps from getting refreshed until they are opened, and lowers the brightness of the screen.
On Android, users can pre-set the amount of battery life that remains when the Battery Saver mode is activated automatically. Some will pick 15% or lower. However, some sporting a handset running Android 9 found that level set at 99% (not by them), which essentially turned the Battery Saver mode on instantly on these devices.
As Google explained on the Google Pixel Reddit page, this issue was accidentally caused by "an internal experiment to test battery saving features that was mistakenly rolled out to more users than intended." Google has since returned the Battery Saver mode to default settings. And while Google did say it was sorry, some of those affected were upset that Google had changed settings on their handsets without notifying them. But what these users aren't grasping is that they weren't supposed to be involved in the experiment in the first place, so there was no reason for Google to alert them about it in advance.
So with everything returned to normal, you shouldn't worry anymore about the Battery Saver mode activating randomly on your Pie powered Android phone.