When I created a Data Import job, and tried to preview it, it errored becayse the datetime mask expression was incorrect. I went back into editing the job, and fixed that and clicked save, but in doing so it had added two more column definitions to the job, namely Columns 6 and 7. That meant that it was going to treat columns 6  as both the accumulating data column AND a Status Data Column, and the Column 7 as a Tags Column. While I should not hit save probably before I correct this, the fact that it inserts new column definitions other than the ones I had just saved is an extra opportunity for error. In the screenshot I had to manually clear out those columns, even though I had never defined them before.