The macOS installation I never prompted, couldn't be completed My MacBook Pro is running High Sierra at the moment and I have been pretty bothered by the Mojave notifications in last weeks, which I have always ignored because I don't want to update at this moment.

Yesterday I got the usual notification asking me to perform "updates". I said "try tonight". Nowhere in my mind I would say that this includes an OS update, and I think everyone can agree?

Got to bed, woke up (late morning) and the machine was not displaying anything - just the touch bar lit. So after a while I forced a shutdown.

When restarting, I got a The macOS installation couldn't be completed dialog, with a console showing the crash logs. At this point my mind was just a huge "What the...?!" and reboots did not solve the problem - macOS would try to perform this installation again, and fail.

There was an option to save the log file and I did, here it is. The important part seems to be

Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Crashed Thread:        0  Dispatch queue: com.apple.main-thread
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]:  
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000018
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Exception Note:        EXC_CORPSE_NOTIFY
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]:  
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Termination Signal:    Segmentation fault: 11
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Termination Reason:    Namespace SIGNAL, Code 0xb
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: Terminating Process:   exc handler [0]
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]:  
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: VM Regions Near 0x18:
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]: --&gt; 
Feb 17 11:14:00 MacBook-Pro ReportCrash[551]:     __TEXT                 000000010f04c000-000000010f063000 [   92K] r-x/rwx SM=COW  /System/Installation/CDIS/macOS Installer.app/Contents/MacOS/macOS Installer


I got out of this mess by choosing Startup Disk among the options and selecting Macintosh HD.

I have a few questions and would appreciate any help.


Why did this happen? Is it really because of my "try tonight" to the updates notification? Isn't this crazy? I have seen now that Safari was updated, and that's the kind of update I was expecting.
[Related] How can I prevent this from happening again?
When I will be ready to upgrade, what should I do in order for the installation to NOT fail like it did?
I can't answer all your questions, but we are seeing this issue fairly commonly in our workplace, with (I think) one specific update.  The fix is as you describe - choose the Startup Disk from the Utilities menu, select the main HD, and reboot.  It doesn't happen again.