New fresh install on a mac mini M2 installs macOS Tahoe beta 26.0?

[Thomas J]

Helped my brother today (he's 69 years old) to install his newly purchased used Mac mini M2 8/512. The computer was completely erased so it behaved like a new fresh install. He has previously newer owned a Mac, iPad or iPhone but had an old (+15 years) Apple account that was probably used by one of his children (he didn't remember). So it was a bit of a hassle to look up the password etc. But everything went well. To my surprise, I didn't recognize the operating system when the installation was complete. It didn't look like macOS Sequoia, so I checked the system settings and it said macOS Thaoe beta 26.0. What the .... has anyone else experienced this?

 

[Bigwaff]

Helped my brother today (he's 69 years old) to install his newly purchased used Mac mini M2 8/512. The computer was completely erased so it behaved like a new fresh install.

Purchased used. Probably had macOS 26 beta installed previously. The only way to "downgrade" is to DFU restore.

How to revive or restore Mac firmware - Apple Support

Rarely, a Mac with Apple silicon or the Apple T2 Security Chip might stop responding and need its firmware to be revived or restored by another Mac.

support.apple.com

To my surprise, I didn't recognize the operating system when the installation was complete.

The "installation was complete" ... the "installation" being what process exactly?

 

[DeltaMac]

If the Mac mini was simply reset, then it would appear to be newly installed (just as taking a new Mac out of the box, and setting it up.) The previous owner (or the shop where it was purchased used) could have simply chose to Erase all Content and Settings (just by choosing that in System Settings). The system is NOT deleted, just anything connected to a user, and would appear the same as first power on after doing a full erase and install, or same as the first boot after opening up a new Mac. And, if the previous owner had installed a Tahoe beta, the normal Erase all Content and Settings does not remove the Tahoe system, just everything else.
If you really want to go back to an older macOS system, then, as Bigwaff said, you will need to connect to a second Mac to do a DFU restore to an older system.
(Final release of Tahoe should be middle of next week, if the Mac is working OK with the beta