Your granny might have become a Windows 10 Insider yesterday after a Microsoft snafu saw non-Windows 10 Insiders being offered a new Windows 10 Insider Preview Build.
The invite was delivered via a notification which can be seen above. Brandon LeBlanc, a member of the Windows 10 Insider team, has acknowledged the issue and said only a limited number of people were affected, and that Microsoft stopped the process as soon as they were informed of the issue.
I can look at the data and see how many PCs received the notification. Despite the reports you are seeing here, it's not wide spread. Also – we turned off the notifications for the time being to investigate.
— Brandon LeBlanc (@brandonleblanc) August 31, 2018
Despite being notified of the new build, users were thankfully not able to download the new software without joining the program, though of course, this is relatively easy with only a few clicks these days.
The current Windows 10 RS5 builds are now pretty mature and polished, so of course, this would not be a complete disaster, and the incident itself may be a sign that Microsoft is getting ready to push the October 2018 Update to regular users soon.