>> I wish I could describe exactly what happened.  I found
>> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=70541 which sounds
>> very much like what I'm seeing, but I don't use any input method.

Thanks for your description of the problem.  A reproducible recipe is of
course better, but sadly, we can't always have what we want.
Let's hope we find that eventually we'll get what we need.

>> Eventually, this pops into *Messages*:
>> 
>> #+begin_quote
>> cl--assertion-failed: Assertion failed: (or track-changes--before-no (<=
>> track-changes--before-beg (track-changes--state-beg track-changes--state)
>> beg end (track-changes--state-end track-changes--state)
>> track-changes--before-end))
>> #+end_quote

Hmm... this clearly points to a bug in track-changes.

>> #+begin_quote
>> ⛔ Warning (emacs): Missing/incorrect calls to ‘before/after-change-functions’!!
>> Details logged to ‘track-changes--error-log’
>> #+end_quote

Barring bugs in track-changes, this points to a bug in some other part
of Emacs, which prevents track-changes from getting the info it needs.
Track-changes (and Eglot) are supposedly equipped to deal with such
problems, because we know we still have many of them.

>> Over time, they just keep repeating,

Hmm... `track-changes--recover-from-error` should (as the name suggests)
"recover" so they shouldn't just keep repeating, unless the source of
the problem keeps repeating as well.  Or unless there's a bug in
track-changes.

>> although I think killing the
>> buffer and reopening the file makes it stop.
>>
>> If I have inspected ‘track-changes--error-log’ correctly, this is
>> what it contains (looks like some binary, hope it comes through
>> in some useful fashion or another):

It's a list of debug info, one per "recovery".  Each debug info is made
of the buffer name, the inconsistency encountered, then a backtrace, and
an extract of (raw) `view-lossage`.  Here's your data reformatted:

    (("lib.rs" (buffer-size 5254 5218)
      ((t track-changes--recover-from-error ... nil)
       (t track-changes-fetch ... nil)
       (t eglot--track-changes-fetch ... nil)
       (t eglot--signal-textDocument/didChange nil nil)
       (t eglot--signal-textDocument/didSave nil nil)
       (t run-hooks ... nil)
       (t basic-save-buffer ... nil)
       (t save-buffer ... nil)
       (t funcall-interactively ... nil)
       (t call-interactively ... nil)
       (t command-execute ... nil))
      [111 (nil . other-window)
       103 (nil . recompile)
       121 (nil . undefined)
       24 96 (nil . next-error)
       1 (nil . move-beginning-of-line)
       11 ...])
     ("lib.rs" (buffer-size 5256 5254)
      ((t track-changes--recover-from-error ... nil)
       (t track-changes-fetch ... nil)
       (t eglot--track-changes-fetch ... nil)
       (t eglot--signal-textDocument/didChange nil nil)
       (t eglot--signal-textDocument/didSave nil nil)
       (t run-hooks ... nil)
       (t basic-save-buffer ... nil)
       (t save-buffer nil nil)
       (t #[257 "r\211q\210\300 )\207" [save-buffer] 2 ...]
       (t map-y-or-n-p ... nil)
       (t save-some-buffers ... nil)
       (t recompile ... nil)
       ...)
      [19 (nil . save-buffer)
       24 24 (nil . exchange-point-and-mark)
       23 (nil . kill-region)
       24 19 (nil . save-buffer)
       backspace (nil . delete-backward-char) ...])
     ("lib.rs" (buffer-size 5278 5256)
      ((t track-changes--recover-from-error ... nil)
       (t track-changes-fetch ... nil)
       (t eglot--track-changes-fetch ... nil)
       (t eglot--signal-textDocument/didChange nil nil)
       (t eglot--signal-textDocument/didSave nil nil)
       (t run-hooks ... nil)
       (t basic-save-buffer ... nil)
       (t save-buffer ... nil)
       (t funcall-interactively ... nil)
       (t call-interactively ... nil)
       (t command-execute ... nil))
      [(nil . backward-word)
       2 (nil . backward-char)
       67108896 (nil . set-mark-command)
       134217734 (nil . forward-sexp)
       23 (nil . kill-region)
       24 19 (nil . save-buffer)
       ...])
     ("lib.rs" (buffer-size 5324 5278)
      ((t track-changes--recover-from-error ... nil)
       (t track-changes-fetch ... nil)
       (t eglot--track-changes-fetch ... nil)
       (t eglot--track-changes-signal ... nil)
       (t #[771 "\211^BZ\211^HG^E] ...)
       ...)
      ...)
     ...)

This suggests the error is always detected "late" when we find that the
buffer-size is not the one we expected, so some buffer change presumably
happened without informing track-changes.
That doesn't explain the assertion failure.

Also, I don't see anything "unusual" in the view-lossage.

>> I wish I could reliably repeat it.  Given that it occurs after
>> many hours of work, `emacs -Q` isn't an option.

Thanks.
I stared at the code for a bit trying to see how that assertion failure
can occur, but nothing came to mind yet.  I think we'll need more data.

Can you try to install the patch below, then run with
`debug-on-error` enabled?
Hopefully when the error occurs, this will give us more info (ideally
including a backtrace).


        Stefan