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INVALID_FIELD_FOR_INSERT_UPDATE error with standard object Lookup field on standard object?

I have a custom field of a standard object on a standard object.  In this case,  "Contact__c" (Lookup relationship to Contact obj) is on the Opportunity object.  When using the api to create a new Opportunity, I supply a valid Id for a Contact into the Contact__c field and receive the INVALID_FIELD_FOR_INSERT_UPDATE error.  API documentation states that this occurs when "The specified change to a person account record type cannot be combined with any other field update.
This is not the case, as I am merely trying to create a new Opp. 
Am I missing something?
I recalled seeing another thread in which the developer was seeing this error when going against PE where the error occurred b/c the field was not on a page layout.  I believe it was Simon who alerted him to this, but said it should work in EE/UE even if it's not on a page.  Apparently, it does still need to be on a page, as it works now that I added it.
I think, most importantly, that this should be updated in the API docs, as the specific error description is a little misleading, or, actually, just a little too limited.