When trying to change my details (by clicking on the link that says to take a squiz at mydetails or something similar) after ordering a ticket to the same same Christmas party I got this error message:

ERROR: get_template() failure: [/websites/samesame/exports/1.6.3_php5_21435/samesame/_fasttemplates//auth/pageheading_membership.tmpl] Bad arguments.

So I had to update my details via my profile page then go back to the shop to complete the purchase.

Just letting you guys know

Thanks for pointing this out, will bug it.