Reproducible with 'emacs -q':
* In a freshly started Emacs, open an archive, e.g. a .tar.gz file in a buffer.
* Switch to another buffer, type a couple of letters and then start pressing 'M-/' (dabbrev-expand).
* It is very likely you will hit a "very useful" dabbrev in binary form taken from the archive. E.g. I opened a large tarball, then switched to *scratch*, typed "tz", and the very first dabbrev I got was "tz\327r".
Request: Emacs should ignore archive and other binary buffers when generating dabbrevs. There are some settings to ignore buffers by name, but this would rather ignore buffers by mode. Also, this should be the default because I cannot imagine binary dabbrevs being useful to more than maybe 0.01% of users.