Nailed it! If *while there is no table defined for the worksheet*, a validation (type: 'Enter list of items', 'Allow invalid data' unticked) is put on a header cell (because someone for convenience did the entire column, say), then when a table is defined on that worksheet, the insertion of the table's <entry/> to the feed will fail with "403 forbidden: strictValidationFailed", probably because it's trying to write an 'invalid' value into the header cell.
However, if the validation is put on the header cell while there is a table defined for the worksheet, it's all fine - removing the table definintion and inserting a new one copes with the situation by automatically dropping the validation from the header cell in question. Subtle. PS. (The 'new validation' vs 'old validation' distinction I made above probably doesn't hold. I saw all sorts of sizes of menus in all sorts of situations just now, and couldn't really explain why I got one menu and not another - it seemed not to have to do with when the validation was applied to a cell, though).
