If browse-url-browser-function is defined as a list of pairs
(REGEXP . FUNCTION), like:
(setq browse-url-browser-function '(("i\.imgur\.com" browse-url-emacs)
                                    ("." browse-url-chromium)))
browse-url (with an "i.imgur.com" url) will print an error saying that
"(invalid-function (browse-url-chromium))". This is probably because of
how browse-url handles such pairs:

;; The `function' can be an alist; look down it for first match
;; and apply the function (which might be a lambda).
(catch 'done
  (dolist (bf function)
    (when (string-match (car bf) url)
      (apply (cdr bf) url args)
              ^^^^^^ (cdr bf) returns (browse-url-emacs) instead of browse-url-emacs
      (throw 'done t)))

This can be easily fixed by replacing "(cdr bf)" with "(cadr bf)". I
have attached a patch which does the same. If it looks good, I'll push
it to master.