Erc blows out in Emacs 25.0.50 with the change to subword-mode. See https://debbugs.gnu.org/db/17/17558.html

erc-subword-fix-emacs-HEAD.patch

From 3017aba1093c8f57d3a3b7193692d62a56b68d3b Mon Sep 17 00:00:00 2001
From: Dima Kogan <dima@HIDDEN>
Date: Tue, 30 Dec 2014 23:29:21 -0800
Subject: [PATCH] ERC no longer gets confused by subword-mode

In commit 6ddc44225e743e2b2a0d5c192f50aefd7a4a915b subword-mode was
integrated into the syntax table instead of simply remapping the
interactive motion bindings as was done previously.  This had the
unintended effect of changing the behavior of lisp programs that touch
words.  In the case of ERC, it completely broke it: emacs now throws an
error when ERC is launched, making it unusable when subword-mode is
active.

This commit replaces the word-oriented calls with ones that navigate
the buffer using syntax classes

Closes: #17558
---
 lisp/erc/erc-backend.el | 11 ++++++++++-
 lisp/erc/erc-button.el  | 30 ++++++++++++++++++++++--------
 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/lisp/erc/erc-backend.el b/lisp/erc/erc-backend.el
index 1ef494c..0e80438 100644
--- a/lisp/erc/erc-backend.el
+++ b/lisp/erc/erc-backend.el
@@ -480,7 +480,16 @@ Currently this is called by `erc-send-input'."
   (with-temp-buffer
     (insert str)
     (goto-char (point-min))
-    (upcase-word 1)
+
+    ;; this is (upcase-word 1), but working even with subword-mode
+    ;; active
+    (skip-syntax-forward "^w")
+    (let*
+        ((word-start (point))
+         (word-end
+          (progn (skip-syntax-forward "w") (point))))
+      (upcase-region word-start word-end))
+
     (buffer-string)))
 
 (defun erc-server-setup-periodical-ping (buffer)
diff --git a/lisp/erc/erc-button.el b/lisp/erc/erc-button.el
index 10e7318..d194627 100644
--- a/lisp/erc/erc-button.el
+++ b/lisp/erc/erc-button.el
@@ -300,14 +300,28 @@ specified by `erc-button-alist'."
     (when (or (eq t form)
               (eval form))
       (goto-char (point-min))
-      (while (forward-word 1)
-        (setq bounds (bounds-of-thing-at-point 'word))
-        (setq word (buffer-substring-no-properties
-                    (car bounds) (cdr bounds)))
-        (when (or (and (erc-server-buffer-p) (erc-get-server-user word))
-                  (and erc-channel-users (erc-get-channel-user word)))
-          (erc-button-add-button (car bounds) (cdr bounds)
-                                 fun t (list word)))))))
+
+      (while
+          (progn
+
+            ;; I move forward a word (independent of subword-mode) ...
+            (skip-syntax-forward "^w")
+            (let*
+                ((word-start (point))
+                 (word-end
+                  (progn (skip-syntax-forward "w") (point))))
+
+              ;; ... if the word was empty we're at the end of buffer ...
+              (and (/= word-start word-end)
+
+                   ;; ... otherwise, we do stuff with this word
+                   (progn
+                     (setq word (buffer-substring-no-properties
+                                 word-start word-end))
+                     (when (or (and (erc-server-buffer-p) (erc-get-server-user word))
+                               (and erc-channel-users (erc-get-channel-user word)))
+                       (erc-button-add-button word-start word-end
+                                              fun t (list word)))))))))))
 
 (defun erc-button-add-buttons-1 (regexp entry)
   "Search through the buffer for matches to ENTRY and add buttons."
-- 
2.1.3