Let's not forget that this is the man in charge at the Department of Work and Pensions. This means he is the man at the top in a department that currently has a major campaign going on about benefit fraud. To claim a 'clerical error' the first time was incredulous, but to then admit a week later that you forgot that you forgot about others when you were remembering about the first makes you look at best incompetent and at worst a corrupt fraudster.
No ifs. No buts. No excuses. Time to go.