Complex sentence: Poirot's secretary, Miss Lemon, has few human weaknesses and the only two mistakes she is ever recorded making are a typing error during the events of Hickory Dickory Dock and the mis-mailing of an electric bill.
Split into simpler sentences: Poirot's secretary, Miss Lemon, has few human weaknesses. The only two mistakes she is ever recorded making are a typing error in "Hickory Dickory Dock" and the mismailing of an electric bill.