Emacs 23.0.60.1, CVS from May 14, 2008.  I get the following sorts of
errors at random intervals: (In this case, it occurred when opening a
message in Wanderlust.)  I would be grateful for any advice on finding
the source of this problem.

Debugger entered--Lisp error: (error "Invalid image specification")
  image-mask-p((holiday-hebrew-rosh-hashanah nil "23.1"))
  #[(bind) "Ä‰\n£¢Å==r =\n8:ƒ == =\n›‰=r ==\"‰=r 	:ƒr 	@Ã==r =	@!ƒr = „E ==r =	!‰=S =!„X ==\"=\n!=\n8:ƒg ==h =\n›==#ˆ)\n*‡" [plist image-exp bind image nil menu-item 4 5 plist-get :image fboundp display-images-p eval image-mask-p append (:mask heuristic) copy-sequence plist-put] 5]((new-file menu-item "Visit New File..." find-file ([24 6] . "  C-x C-f") :enable (menu-bar-non-minibuffer-window-p) :help "Specify a new file's name, to edit the file" :image (tool-bar-find-image (if ... ... ...))))
  mapcar(#[(bind) "Ä‰\n£¢Å==r =\n8:ƒ == =\n›‰=r ==\"‰=r 	:ƒr 	@Ã==r =	@!ƒr = „E ==r =	!‰=S =!„X ==\"=\n!=\n8:ƒg ==h =\n›==#ˆ)\n*‡" [plist image-exp bind image nil menu-item 4 5 plist-get :image fboundp display-images-p eval image-mask-p append (:mask heuristic) copy-sequence plist-put] 5] (keymap (new-file menu-item "Visit New File..." find-file ([24 6] . "  C-x C-f") :enable (menu-bar-non-minibuffer-window-p) :help "Specify a new file's name, to edit the file" :image (tool-bar-find-image ...)) (open-file menu-item "Open File..." menu-find-file-existing (nil) :enable (menu-bar-non-minibuffer-window-p) :help "Read an existing file into an Emacs buffer" :image (tool-bar-find-image ...)) (dired menu-item "Open Directory..." dired ([24 100] . "  C-x d") :enable (menu-bar-non-minibuffer-window-p) :help "Read a directory, to operate on its files" :image (tool-bar-find-image ...)) (kill-buffer menu-item "Close" kill-this-buffer (nil) :enable (kill-this-buffer-enabled-p) :help "Discard (kill) current buffer" :image (tool-bar-find-image ...)) (save-buffer menu-item "Save" save-buffer ([24 19] . "  C-x C-s") :enable (and ... ... ...) :help "Save current buffer to its file" :image (tool-bar-find-image ...) :visible (or buffer-file-name ...)) (write-file menu-item "Save As..." write-file ([24 23] . "  C-x C-w") :enable (and ... ...) :help "Write current buffer to another file" :image (tool-bar-find-image ...) :visible (or buffer-file-name ...)) (undo menu-item "Undo" undo ([31] . "  C-_") :enable (and ... ... ...) :help "Undo last operation" :image (tool-bar-find-image ...) :visible (not ...)) (cut menu-item "Cut" ("Delete text in region and copy it to the clipboard" ... . clipboard-kill-region) :image (tool-bar-find-image ...) :visible (not ...)) (copy menu-item "Copy" ("Copy text in region to the clipboard" ... . clipboard-kill-ring-save) :image (tool-bar-find-image ...)) (paste menu-item "Paste" x-clipboard-yank :enable (not buffer-read-only) :help "Paste (yank) text most recently cut/copied" :image (tool-bar-find-image ...) :visible (not ...)) (search-forward menu-item "String Forward..." nonincremental-search-forward (nil) :help "Search forward for a string" :image (tool-bar-find-image ...)) (print-buffer menu-item "Print Buffer" print-buffer (nil) :enable (menu-bar-menu-frame-live-and-visible-p) :help "Print current buffer with page headings" :image (tool-bar-find-image ...)) (customize menu-item "customize" customize :image (tool-bar-find-image ...) :help "Edit preferences (customize)") (help menu-item "help" #[nil "Á!‡" [menu-bar-help-menu popup-menu] 2 nil nil] :image (tool-bar-find-image ...) :help "Pop up the Help menu")))
  tool-bar-make-keymap(ignore)
  key-binding([tool-bar] t)