I take back what I wrote about this being fixed in 5.10.10.  It's harder
to reproduce in 5.10.10, because it has some additional checks for
whether the current display can handle images.  But if I view the Alan
Cox email in an X frame, then start up a tty frame and visit the
*Article* buffer, I get the same warnings.

We just ran into a very similar problem while beta-testing MH-E 8.3. 
Recent versions of MH-E insert a graphic into the modeline.  If you
start up a tty frame and visit an MH-E buffer, you get the same (or very
similar) warning as with this issue.  Changing MH-E to call make-glyph
with :file instead of with :data made the problem go away.

I can't reproduce the warning with (Gnus 5.10.10 and) XEmacs
21.5.29+0d436a78c514.  Perhaps this is a bug in XEmacs core code?