Good question! The answer is to remove
user-error from option
M-x customize-option debug-ignored-errors, and remove the entry for
However, IMO the Emacs manual should be transparent about this, and it is not. Even the Elisp manual is not clear about this -- the only hint is this mention in passing, within the description of option
The normal value of this variable includes
user-error', as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, "rarely" is not "never"; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to setdebug-ignored-errors' to `nil'.
FWIW, I've submitted Emacs bug #18005 for the missing doc.