Utterance: "I'm Bobby's son."
Target: "I'm Bobby's father."
Discussion: This is a nice, straight-forward semantically mediated lexical slip. There's no need to go mucking through the subconscious mind for a Freudian explanation when the simpler explanation is available. 'son' and 'father' are words that are closely related in meaning. Freudian slips can occur, but you should have a strong case before invoking that sort of thing.