If a donor selects a payment submethod on the credit card form, then goes back to donatewiki and clicks on the bank transfer form, they end up with a method/submethod combination that doesn't match any forms. This causes us to log a bogus error (T106050) and choose a fallback form.
When we have a method in session and a different method comes in on the request, we should purge any submethod we have in session.
This will be annoying in the current setup since we mush all data together before the normalize step. Another argument for @K4-713's idea to tag every data element with its source.