On a page called "My page", the macro

creates the HTML

<a name="Mypage-foobar"></a>

But the link

My page#foo-bar

generates a link to

http://servername/confluence/display/MYSPACE/My+page#Mypage-foo-bar

which is invalid because of the extra '-'.

Casual testing suggests that this can be replicated with other non-alphanumeric characters. Some of these characters may be legal for anchor names.