I just updated to latest git, and it looks like these two changes:

  b12f45e int, nnimap: remove to-buffer in request-head
  04ef222 gnus-int: add to-buffer parameter to gnus-request-head

which were intended to be a change and a revert, ended up doing:

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 1bf8444..9e1cc6e 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,14 @@
+2012-08-06  Julien Danjou  <julien@danjou.info>
+
+	* nnimap.el (nnimap-request-article): Remove to-buffer argument.
+
+	* gnus-int.el (gnus-request-head): Remove to-buffer argument, only
+	supported by nnimap actually. Reverts previous change.
+
+	* gnus-int.el (gnus-request-head): Add an optional to-buffer parameter
+	to mimic `gnus-request-article' and enjoy backends the nn*-request-head
+	to-buffer argument that is already supported.
+
 2012-08-05  Julien Danjou  <julien@danjou.info>
 
 	* gnus-demon.el (gnus-demon-add-handler, gnus-demon-remove-handler):
diff --git a/lisp/nnimap.el b/lisp/nnimap.el
index ebd268e..e195b42 100644
--- a/lisp/nnimap.el
+++ b/lisp/nnimap.el
@@ -597,7 +597,7 @@ textual parts.")
 		(nnheader-ms-strip-cr)))
 	    (cons group article)))))))
 
-(deffoo nnimap-request-head (article &optional group server to-buffer)
+(deffoo nnimap-request-head (article &optional group server)
   (when group
     (setq group (nnimap-decode-gnus-group group)))
   (when (nnimap-change-group group server)
@@ -610,7 +610,7 @@ textual parts.")
 	 article (format "UID FETCH %%d %s"
 			 (nnimap-header-parameters)))
 	(let ((buffer (current-buffer)))
-	  (with-current-buffer (or to-buffer nntp-server-buffer)
+	  (with-current-buffer nntp-server-buffer
 	    (erase-buffer)
 	    (insert-buffer-substring buffer)
 	    (nnheader-ms-strip-cr)


which results in the following when using delete.


nnimap-request-move-article: Wrong number of arguments: #[(article &optional group server) "ƒÅ!Æ	\"…ArÇ qˆ
;ƒÈ	
#
…@É
ÊËÌ \"\"ˆprqˆÍ ˆÎ!ˆÏ ˆ
*B)‡" [group server article buffer nntp-server-buffer nnimap-decode-gnus-group nnimap-change-group nnimap-buffer nnimap-find-article-by-message-id nnimap-get-whole-article ...] 5], 4