Don't prevent contact with Cancelled membership from signing up online
Contribution page prevents signing up for a membership if the matched contact has an existing cancelled membership of the same type.
I don't really understand why https://github.com/civicrm/civicrm-core/pull/3531 ever got merged given that it seems no-one was in favour of restricting memberships in this way: https://issues.civicrm.org/jira/browse/CRM-14645
A restriction such as this could easily be put in place via the validateForm
hook for anyone who actually needs it and it seems a pretty obscure restriction to have hardcoded in core.