What is the “Record write validation failed. DataKey” error?

        
During Enterprise Portal Development, I received this error in a Tunnel/Wizard that I put together to create a new record in a custom table. I originally googled this error when I first received it, but absolutely no results were returned. Not too long after, a little debugging revealed that the problem was that I was not supplying the value for a mandatory (Number Sequence) field.

If I come across other causes for the problem, I will append them to this blog as I find them.