I recently upgraded to 1.5.6.1 and now I'm seeing this error when I attempt to update a product.
Unknown column 'a.start_day' in 'where clause'
Attached is a screen capture of the full error.
Edit: Actually I just realized, this happens as soon as I log into Prestashop, provided that I have debugging enabled with
define('_PS_MODE_DEV_', true);