The Google Home Mini isn't even being shipped yet, but one of the review samples sent to Artem Russakovskii from Android Authority had a serious bug with it that was recording sounds in his house 24/7.
He noticed the bug because the device would flash its lights even if he hadn't issued the OK Google command. He was able to work out what was going on by logging into the activity portal and finding all the recordings that were stored on Google's server.
The issue appears to be with the touch-sensitive button on the top of the speaker that you can use instead of the OK Google command. It is falsely triggering in this unit. Unfortunately, capacitive touch buttons require a bit of calibration to get things working right and they can sometimes be overly sensitive.
Thankfully Google resolved this issue within 3 hours of Artem reporting it by sending out a software patch that completely disabled the button. It is expected they will try and push out a full fix for when the product starts shipping on October the 19th.
While it is commendable that Google carried out this patch so promptly and was open about it, it could also be due to the fact a leading blogger exposed the issue. It also once again raises the privacy issues of having these devices in your home.