This is a further development in this "saga".
After originally asking the question below (which I will delimit with dashes) a number of months ago, I did get a reponse from Wolfram Technical Support:
"The user report you sent us indicates you were auto-completing a symbol which does not exist in our normal contexts."
This led me to switch my keyboard. (The crashes had seemed to occur "instantaneously" upon pressing certain keys. So instantly that it was hard to know exactly what key had been pressed.)
This change of keyboard seemed to resolve the crashing problem--as I've been running many programs in the intervening few months.
However, a few days ago, it seemed to start recurring--with a "vengeance" (and I have sent the error report to Wolfram Technical Support).
The only possible explanation that occurs to me presently is that a few days ago a security update was installed on my Mac. (A keyboard switch now doesn't seem to work in eliminating the problem.)
I'm so traumatized that I've just been working with Version 11 (for which this never seems to happen), rather than 12. (Needless to say, 12 can run indefinitely [when no further input is made].)
Maybe this will still be considered "off-topic".
I had Mathematica 12 installed ten days or so ago (and just now re-installed) on my university computer. Although I do seem to be able to run sustained programs for a day or so, it has also totally unexpectedly crashed on various occasions, even running a few basic lines of commands.
I'd written to email@example.com, but only received as a reply so far:
"In order to better diagnose the issue, please provide more information such as:
-A simple code sample of, say, 10-20 lines that essentially illustrate the issue.
-A screenshot of any windows that show up.
-Any error messages you see when you try to open Mathematica."
Hard to do if everything disappears.
I've been using Mathematica intensively for decades now, and have never encountered a problem remotely similar to this.
I've also been running version 12 at home, and this phenomenon has not occurred there.
Guess I'll just use Version 11 on campus (for which I've never had such a problem) until this matter resolves itself.
This is my "hardware overview":
Model Name: Mac mini Model Identifier: Macmini4,1 Processor Name: Intel Core 2 Duo Processor Speed: 2.4 GHz Number of Processors: 1 Total Number of Cores: 2 L2 Cache: 3 MB Memory: 4 GB Bus Speed: 1.07 GHz Boot ROM Version: 22.214.171.124.0 SMC Version (system): 1.65f2 Serial Number (system): C07DV0VLDD6L Hardware UUID: 58037265-42DA-53A2-8C8C-EF4D37D3AFB6
In response to the comments, here's the beginning of the Console user report:
Process: Mathematica  Path: /Applications/Mathematica 3.app/Contents/MacOS/Mathematica Identifier: Mathematica Version: 126.96.36.19906964 (12.0) Code Type: X86-64 (Native) Parent Process: ???  Responsible: Mathematica  User ID: 503
Date/Time: 2019-04-30 12:40:21.397 -0700 OS Version: Mac OS X 10.12.6 (16G1918) Report Version: 12 Anonymous UUID: 52BDE4CB-BD76-9870-077B-D24E45CE6A58
Sleep/Wake UUID: 1F6DA416-FBD3-4B99-9E68-426AE388E0DB
Time Awake Since Boot: 320000 seconds Time Since Wake: 840 seconds
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV) Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000078 Exception Note: EXC_CORPSE_NOTIFY
VM Regions Near 0x78: --> __TEXT 000000010dc25000-000000010e9b7000 [ 13.6M] r-x/rwx SM=COW /Applications/Mathematica 3.app/Contents/MacOS/Mathematica
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread 0 com.wolfram.Mathematica 0x000000010e12219c 0x10dc25000 + 5231004 1 com.wolfram.Mathematica 0x000000010df48c10 0x10dc25000 + 3292176
Any more needed? Insightful?