It sounds like there was an invalid value that SFDC could not recognize because the field types in each system aren't aligned.
Check field management and then go to your SFDC Admin or check it yourself in SFDC.
Super odd. It's actually a field that we haven't used in years (created by my predecessor), so I'm surprised that it's suddenly an issue.
No values in the fields, so not sure why Marketo was freaking out. Though that I may was well go through the process of deleting the fields and hiding it from Marketo (we don't use them – why not clean it up a bit?), and Marketo's now giving me new errors!
_retrieveSObjects failed for type 'Contact' - INVALID_FIELD: Organization_Type__c, OwnerId, Phone, Pipeline_Status__c, Primary_Competitor__c ^ ERROR at Row:1:Column:3543 No such column 'Pipeline_Status__c' on entity 'Contact'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names.
Pipeline_Status__c is also a field I deleted / hid, so I'm guessing that it's causing the problem. Just logged a case with Marketo – I'm at a loss of what to do next.
I suspect you need to ask your SFDC Admin about this. This could be a formula field and for some reason SFDC is requiring it or perhaps Marketo is pushing a value to it when it should not.
I would be very, very careful about next steps since you uncovered more errors. Marketo Support may be able to guide you here, but it looks largely on the SFDC side, so it's really your admin's responsibility.