If a page has any punctuation characters in it, then many links created by confluence use the viewpage.action style URL rather than display. Like this:

http://hostname/pages/viewpage.action?pageId=638651081

Unfortunately the "add comment" link does not take this possibility into account, and applies additional parameters introduced by '?' instead of '&'. This doesn't work.

http://hostname/pages/viewpage.action?pageId=638651081?showComments=true&showCommentArea=true#addcomment