How do undo for damage done by accidental decipher-mode?

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Try the command `debug-on-entry`. https://www.gnu.org/software/emacs/manual/html_node/elisp/Debugger.html

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You can also hit`C-h v l`, which will display a log of recent keystrokes and what commands they invoked. I've never even heard of this mode before, I have no idea how this is happening.

I certainly can't guess how it's getting triggered, but after a brief look at the code I can tell you that: * `decipher-mode` is a major mode, and it does indeed kill the undo list for the buffer (it also makes the buffer read-only), so yeah... not ideal to *accidentally* invoke that while editing. * Being a major mode, you should be able to replace it by invoking any *other* major mode, or by calling `M-x normal-mode`. It looks like doing so should re-enable undo (but is not going to restore the original clobbered undo list). Also, `(advice-add 'decipher-mode :override #'ignore)` is a good way to disable a function, as (a) it will survive the original function getting redefined (if for some strange reason something explicitly reloaded `decipher.el`), and (b) it's possible to remove the advice to restore the original function (probably not wanted here, but useful in the general case).

Yep. If OP can't figure out what's triggering it via `debug-on-entry` they could just block it by advicing it to a no-op.

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