What is causing default membership type error?
I think the answer to this was that there was a bug whereby the 'weight' was being used in one situation, whereas the type ID was being used in the other. And this error only showed up if you had changed the 'weight' ie the order of your Membership Types.

To quote the ticket in JIRA


  In the contribution page edit form, the default is selected by membership_type_id. When the contribution form is displayed, it's using the weight of the membership type instead


This has now been fixed by the Fuzion Team.