[noun (person)] + としたことが、[sentence]
としたことが means "of all people": someone you would expect to be careful or competent has slipped up. It follows a noun for a person, very often わたくし私, and the sentence after it names the blunder.
About yourself it is self-reproach; about someone else it is surprise. The sentence usually ends on a word that carries that feeling: 〜~てしまった for regret, 〜~なんて or 〜~とは for disbelief. A modifier before the noun often says why the slip is surprising: あのしんちょう慎重な, ぷろプロの, べてらんベテランの.
The phrase sounds a little dramatic and old-fashioned. In everyday talk the self-reproaching わたくし私としたことが is the form you will hear most.