Today's cvs for 21.5, Redhat 6.2.
No init.el or .emacs, start a fresh xemacs, setup some things through
the menus and custom and save the options so that you get a custom.el
with:
(custom-set-variables
 '(gutter-buffers-tab-visible-p t)
 '(paren-mode (quote blink-paren) nil (paren))
 '(column-number-mode t)
 '(enabled-behavior-list (quote (resize-minibuffer recent-files mwheel jka-compr
func-menu filladapt efs)))
 '(pending-delete-mode t nil (pending-del))
 '(toolbar-visible-p nil)
 '(line-number-mode t)
 '(user-mail-address "galibert(a)pobox.com")
 '(query-user-mail-address nil))
(custom-set-faces
 '(default ((t (:background "white"))) t))
Restart an xemacs, it will fail on startup:
(1) (initialization/error) An error has occurred while loading
/home/galibert/.xemacs/init.el:
Assertion failed: (equal enabled-behavior-list val)
Backtrace follows:
  signal(cl-assertion-failed ((equal enabled-behavior-list val)))
  (or (equal enabled-behavior-list val) (signal (quote cl-assertion-failed) (list ...)))
  (progn (or (equal enabled-behavior-list val) (signal ... ...)) nil)
  (assert (equal enabled-behavior-list val))
  # bind (enable-list disable-list old-val)
  (let* ((old-val enabled-behavior-list) (disable-list ...) (enable-list ...)) (dolist (b
disable-list) (disable-behavior b t)) (dolist (b enable-list) (enable-behavior b t))
(assert (equal enabled-behavior-list val)))
  (if within-behavior-enabling-disabling (set sym val) (let* (... ... ...) (dolist ...
...) (dolist ... ...) (assert ...)))
  # bind (val sym)
  (lambda (sym val) (if within-behavior-enabling-disabling (set sym val) (let* ... ... ...
...)))(enabled-behavior-list (resize-minibuffer recent-files mwheel jka-compr func-menu
filladapt efs))
  # bind (set comment requests now value symbol entry immediate args theme)
  custom-theme-set-variables(user (gutter-buffers-tab-visible-p t) (paren-mode (quote
blink-paren) nil (paren)) (column-number-mode t) (enabled-behavior-list (quote
(resize-minibuffer recent-files mwheel jka-compr func-menu filladapt efs)))
(pending-delete-mode t nil (pending-del)) (toolbar-visible-p nil) (line-number-mode t)
(user-mail-address "galibert(a)pobox.com") (query-user-mail-address nil))
  apply(custom-theme-set-variables user ((gutter-buffers-tab-visible-p t) (paren-mode
(quote blink-paren) nil (paren)) (column-number-mode t) (enabled-behavior-list (quote
...)) (pending-delete-mode t nil (pending-del)) (toolbar-visible-p nil) (line-number-mode
t) (user-mail-address "galibert(a)pobox.com") (query-user-mail-address nil)))
  # bind (args)
  custom-set-variables((gutter-buffers-tab-visible-p t) (paren-mode (quote blink-paren)
nil (paren)) (column-number-mode t) (enabled-behavior-list (quote (resize-minibuffer
recent-files mwheel jka-compr func-menu filladapt efs))) (pending-delete-mode t nil
(pending-del)) (toolbar-visible-p nil) (line-number-mode t) (user-mail-address
"galibert(a)pobox.com") (query-user-mail-address nil))
  # bind (current-load-list)
  # (unwind-protect ...)
  # bind (load-file-name)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  load-internal("/home/galibert/.xemacs/custom.el" t t t undecided)
  # bind (path handler filename nosuffix nomessage noerror file)
  load("/home/galibert/.xemacs/custom.el" t t t)
  load-user-init-file()
  #<compiled-function nil "...(10)" [init-file-had-error
load-user-init-file-p load-user-init-file nil] 2>()
  # (unwind-protect ...)
  call-with-condition-handler(#<compiled-function (__load_init_file_arg__)
"...(26)" [init-file-had-error user-init-file __load_init_file_arg__ errstr
error-message-string message "Error in init file: %s" lwarn initialization error
"An error has occurred while loading %s:\n\n%s\n\nBacktrace follows:\n\n%s\n\nTo
ensure normal operation, you should investigate the cause of the error\nin your
initialization file and remove it.  Use the `-debug-init' option\nto XEmacs to enter
the debugger when the error occurs and investigate the\nexact problem."
backtrace-in-condition-handler-eliminating-handler t] 8> #<compiled-function nil
"...(10)" [init-file-had-error load-user-init-file-p load-user-init-file nil]
2>)
  # (condition-case ... . ((error)))
  # bind (debug-on-error debug-on-error-from-init-file debug-on-error-should-be-set
debug-on-error-initial)
  load-init-file()
  # bind (command-line-args-left)
  command-line()
  # (unwind-protect ...)
  normal-top-level()
  # (condition-case ... . error)
  # (catch top-level ...)
To ensure normal operation, you should investigate the cause of the error
in your initialization file and remove it.  Use the `-debug-init' option
to XEmacs to enter the debugger when the error occurs and investigate the
exact problem.
After the assertion failure, the status of enabled-behavior-list is:
`enabled-behavior-list' is a variable declared in Lisp.
  -- loaded from "/home/galibert/xe/xemacs-21.5/lisp/behavior.elc"
Value: (efs filladapt func-menu jka-compr mwheel recent-files resize-minibuffer)
Documentation:
List of currently enabled behaviors.
Normally, don't set it directly; use `enable-behavior' or `disable-behavior'.
  OG.