In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo version 1.16.0)
 of 2022-02-19 built on orion
Repository revision: 563bb08c5998f82e034a0aa1b48dce29fb9bc375
Repository branch: rgr-smtpmail-env-from
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: openSUSE Leap 15.3

Configured using:
 'configure --with-dbus=no --with-gsettings=no --with-gif=ifavailable
 --with-tiff=no --with-gnutls=yes --with-gconf=no'

Configured features:
ACL CAIRO FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG LIBSELINUX LIBXML2
MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

To reproduce:

   In master ecaedf2117cb015ad4028e4d6fc7058608c98096:

   1.  "emacs -Q &"

   2.  Visit pretty much any random file that is NOT a tags file; for
this purpose, lisp/abbrev.el from the source tree will do nicely.

   3.  Type "C-x C-b" to get the "*Buffer List*" window to appear.

   4.  Type "C-x o" to move the to the buffer list, then move to the
abbrev.el line, and type "t" to invoke Buffer-menu-visit-tags-table on
it (which I seem to do accidentally more often than I care to admit).
This produces the error message:

	user-error: File /scratch/rogers/emacs/lisp/abbrev.el is not a valid tags table

but the abbrev.el buffer is left in tags-table-mode, and any undo
information is thrown away.

   The attached patch addresses the problem in a straightforward way by
making Buffer-menu-visit-tags-table prompt the user for buffers not
already in tags-table-mode.

   The real problem is that visit-tags-table assumes that the user knows
what they're doing, and makes these irreversible changes before being
sure of having a valid tags table.  That would be a harder thing to fix,
though -- and might not deal as well with my buffer-menu typos.  ;-}

					-- Bob Rogers
					   http://www.rgrjr.com/