gubi

There's a thread at MacRumors as well reporting issues for some users.  At least my Mac didn't get bricked, I guess.

https://forums.macrumors.com/threads/warning-macos-catalina-10-15-4-supplemental-update-bricked-my-13-inch-macbook-2018.2230133/

cornchip said:

MacPro said:

No issues on 2012 Mac mini, 2012 15" MBP, 2019 MBP, 2019 MBA, 2020 iMac 5k. 2013 Mac Pro.

Yeah but there’re a bunch of one-posters with bricked machines, so your arguments invalid. 

This is the first time in recent memory that an upgrade has borked my Mac.  I created an account to post a warning, hadn't really seen the need to post here before.

Went fine on my wife's 2020 MacBook Air, but my 2012 iMac got stuck at the gray apple screen for an hour.  Forced a reboot and logged in with my password.  Halfway through the boot process it kicked over to "Reinstall MacOS" and had to do an OS reinstall.  Seems okay now.

A few other folks have had issues (see:  https://discussions.apple.com/thread/251259424?page=1)  I ain't saying it's widespread, but this is the first time I've had a problem like this in a long time.