Jonathan Bernier wrote this in a comment here on this blog, and I thought it deserved wider circulation.
It seems to me that a foolproof theodicy would have to be one in which the innocent never suffer. Therefore it would have to argue that those who suffer are not innocent. Such a theodicy should be morally offensive to any decent person. Therefore a foolproof theodicy should be morally offensive. Therefore it’s not an adequate theodicy, for the purpose of theodicy is to make suffering intellectually and morally comprehensible. Therefore one cannot have a foolproof theodicy.