The message attached below induces an end-of-buffer error & a stack trace.

Signaling: (end-of-buffer)
  fill-region-as-paragraph(972 1099 left nosqueeze)
  fill-region(972 1099 left nosqueeze)
  fill-flowed()
  mm-inline-text((#<buffer " *mm*"> ("text/plain" (charset . "us-ascii") (format . "flowed")) nil nil nil nil nil nil))
  mm-display-inline((#<buffer " *mm*"> ("text/plain" (charset . "us-ascii") (format . "flowed")) nil nil nil nil nil nil))
  mm-display-part((#<buffer " *mm*"> ("text/plain" (charset . "us-ascii") (format . "flowed")) nil nil nil nil nil nil) t)
  byte-code("..." [ignored string-match type throw nil mm-inline-override-p handle 4 "inline" mm-attachment-override-p not-attachment mm-automatic-display-p mm-inlined-p mm-automatic-external-display-p t display split-string mm-handle-media-type "/" "text" text gnus-article-mime-handle-alist id gnus-unbuttonized-mime-type-p gnus-insert-mime-button gnus-article-insert-newline move beg -2 gnus-newsgroup-charset (set-buffer gnus-summary-buffer) ((error)) gnus-newsgroup-ignored-charsets mail-parse-ignored-charsets mail-parse-charset mm-display-part mm-insert-inline mm-get-part gnus-treat-article gnus-article-mime-handles 1] 7)
  gnus-mime-display-single((#<buffer " *mm*"> ("text/plain" (charset . "us-ascii") (format . "flowed")) nil nil nil nil nil nil))
  gnus-mime-display-part((#<buffer " *mm*"> ("text/plain" (charset . "us-ascii") (format . "flowed")) nil nil nil nil nil nil))
  gnus-display-mime()
  gnus-article-prepare-display()
  gnus-article-prepare(99 nil)
  gnus-summary-display-article(99 nil)
  gnus-summary-select-article(nil force)
  gnus-summary-show-article(nil)
  call-interactively(gnus-summary-show-article)