diff --git a/django/middleware/common.py b/django/middleware/common.py
index 763918878a..026b3cbc92 100644
--- a/django/middleware/common.py
+++ b/django/middleware/common.py
@@ -39,6 +39,8 @@ class CommonMiddleware:
         # trailing slash or a file extension.
         if settings.APPEND_SLASH and (old_url[1][-1] != '/') and ('.' not in old_url[1].split('/')[-1]):
             new_url[1] = new_url[1] + '/'
+            if settings.DEBUG and request.META['REQUEST_METHOD'].lower() == 'post':
+                raise RuntimeError, "You called this URL via POST, but the URL doesn't end in a slash and you have APPEND_SLASH set. Django can't redirect to the slash URL while maintaining POST data. Change your form to point to %s%s (note the trailing slash), or set APPEND_SLASH=False in your Django settings." % (new_url[0], new_url[1])
         if new_url != old_url:
             # Redirect
             if new_url[0]: