Sometimes the new users get activated in Joomla! without admin approval, even if the relevant option is enabled.
Testing seems to indicate that the activation link included in the message sent to the user is executed.
When the user tries to use it he gets an error message, since the activation one time token has already been consumed.
It has been suggested that some antispam services could do that.
This is more a Joomla! issue, but since it happened with test users associated with the developers' email (corejoomla.com domain), I believed it could be useful to point out.
(the mail server used by joomla!does not cause the problem with other domains).