This problem was first noted here for past versions of moodle, but was closed as inactive/ resolved:

http://tracker.moodle.org/browse/MDL-18745

It seems to still be an issue with Moodle version 2.2

"The email registration confirmation cannot proceed when there is an invalid character in the username.

The signup page allows the username to be entered with in one or more trailing periods (and any name fields as well, not sure if that leads to other related problems)

e.g. username1.

The resulting email registration link in the confirmation email sent for newly added users does not work if the username ends in one or more periods. I'm assuming that the period is stripped because it was invalid.

The moodle error message presented from confirm.php is "Invalid confirmation data"

Since the username is used to create the confirmation email message, there should be validation on the username field prevent entry of invalid characters causing invalid email messages links being sent, which are unusable."