Hello David, there is an unintended behavior in Mathematica 12.1.1 in which if a text cell contains a word not in Mathematica's dictionary (so it has a red underline) certain characters that trigger behavior at the start of a cell (i.e. = opening the Natural Language parser, > opening the python interface) will trigger that behavior when entered in the middle of an input cell.

The immediate workaround is to disable the spell-check in Mathematica's preferences under 


Preferences 
>Interface > Check spelling as you type


We hope to have this behavior fixed soon, but this should serve as a workaround in the meantime.

There is a new build of Mathematica (M-OSX-L-12.1.1-6958981 for macOS) available in the User Portal that fixes this issue. I Imagine that there are new builds for Windows and Linux, but I have no way to tell.

It appeared on 6/23, so If you DLed earlier, the new one is the one to get.

I tested it, and the issue has been resolved (with the usual caveats -- only checked macOS)

Wolfram typical does not use beta testers for 'minor' updates. Perhaps they should.

I have been participating in beta tests since version 3. It is hard to catch everything, especially since the list of things to test for primarily consists of new functionality. However, this incident gives me a clue for the next beta test (12.2?, 13? or if they have Triskaidekaphobia, 14?)

I would imagine that many of the changes will be to ensure compatibility with macOS Big Sur and the release next year of Apple Silicon Macs, plus the usual bells and whistles. I would settle for fewer bugs and better integration with my hardware (I use macOS).

So I guess I assumed Wolfram was depreciating the desktop and trying to go mobile first, as so many other companies have done. What I found after taking a deep breath is that I can create a new file in 12.1.1 and enter a "=" assignment as usual and "[Ctl]=" Euler evaluates the normal entry for Leonard E. However, that is not what happens to any of my saved (100's) of nb files. Entering "=" converts the input cell into some alpha thingy that cannot be changed or evaluated. Entering "[Ctl]=" Euler returns a not-parsed error in some files and the normal output in other files.

I can create new input in the new nb file created under 12.1.1 then paste the expression(s) into an older file (say from Monday) and evaluate it. I can also write code in a text editor and paste it into a nb, then evaluate it correctly, so Mathematical is not brain dead, just unable to accept a typed assignment.

A work-around, such as "(2+w)/.w->3" generates another good result I've not seen before:

This was Mathematica install thirty-something for me, so I did it the same way I that has worked for decades. Thinking of uninstalling Mathematica with CleanMyMac and downloading 12.1.1 again. If that doesn't work, I'll try to find a file fixer utility or something...

To clarify, the presence of certain words in text cells causes input cells in the same notebook to exhibit this behavior.

Yes, we will.

Indeed, the builds for all three platforms (macOS, Windows and Linux) were replaced on 6/23 with ones that have a fix for this issue.

WRI should use semantic versioning for Mathematica.

I share your concern about macOS support. It's no secret that Theo Gray was largely responsible for continued Mac support during the 'dark days', and the macOS seems to be getting a bit less love since he has moved on.

I did find an issue with Big Sur that indicated that the modernization of the front end is incomplete. This problem, while not trivial, is easy to fix. However, I will look with great interest about the transition to Apple Silicon. As you may recall, Wolfram (and Theo Gray) were featured in the transition to Intel, so I had no doubts that Mathematica would be ready for the transition.

My main concern is not with basic functionality, which, I am sure, a recompile with a magic switch will take care of. Mathematica currently does not make full use of Apple GPU support, which makes it harder to accelerate Neural nets and other technologies. With the switch to Apple Silicon, we will undoubtedly get custom GPUs. These will be (probably) much better, but will require the use of Apple's Frameworks to work properly. Wolfram lagged behind support of Metal for graphics, and I fear that they will also all behind in support of Apple's other frameworks. Open source is all well and good, but even the best supported open source projects can't compete with the resources Apple has.

It will be interesting. I have a newish MacBook Pro that will get me through the transition, but I do hope to get a new MacBook sometime in the next year or so.

My feeling is that with more user feedback, Wolfram may be persuaded to adjust their priorities.

Interesting times....