How to debug erratic specializer behavior of generic functions

[Michael Heerdegen]

Hello,

I extended register.el with new register content types.  This involves a
lot of method definitions like

  (cl-defmethod register-val-jump-to ((val TYPE) arg))

where TYPE refers to some of my self defined container types for
register contents.

My problem: the dispatching of these methods is often erratically
broken.  The methods then are not used although their type matches.  The
according test for VAL being of the TYPE, e.g. by using the according
type predicate, succeeds.

This happens randomly.  Sometimes it's broken from the beginning of a
session, sometimes not, sometimes it breaks later while working.  When
it happens, sometimes not all types are affected.  Sometimes reloading
my init file fixes the problem, sometimes not.  I don't see a clear
pattern.

Any hints for where to dig, how to debug, or what could be involved?  So
far I have nothing for emacs -Q, no recipe, that demonstrates the
behavior (that's why I chose to ask here first before starting a bug
report).

TIA,

Michael.