In the short story Sing a Song of Sixpence, Martha is housemaid of Lily Crabtree, an old lady who was bludgeoned to death by a paperweight under circumstances where it appeared that no one had come to the house from outside and yet all the other occupants of the house could not have had the opportunity.
Martha was intensely loyal to her employer, having worked for Miss Crabtree for some 30 years. She told Sir Edward Palliser, investigating the case, that years ago, she had gotten into trouble (euphemism for getting pregnant outside of wedlock or having an illegitimate son in this case). Miss Crabtree had stood by her and taken her back into service when it was over. Martha said she would be willing to die for her mistress. The other occupants of the house, Miss Crabtree's relatives, also considered her absolutely reliable.
Martha told Sir Edward that all the occupants of the house, the four relatives of Miss Crabtree, did not have the opportunity to commit the murder. Three were upstairs and could not have descended without her hearing them because the stairs creaked. The fourth was in a downstairs room but typing the whole time. She was somewhat naive and did not perceive the underlying tensions in the house, believing that all the relatives deep down really liked Miss Crabtree. Nonetheless, Sir Edward, in questioning her, found her a reliable witness and essentially truthful. However when it came to whether someone had come to the house, she appeared to be holding something back.