In Star Wars Episode V: The Empire Strikes Back Luke encounters Boba Fett. Fett detects Luke unholstering of his blaster and later opens fire on Skywalker.
Those that are pointing out that Fett knew Luke would be coming, that is correct. Fett was in the carbonite chamber when the office told Vader an X-Wing was on approach. It is likely Fett knew in general where Luke would be and when he hears Luke unholster his blaster he gets a lock on his location.
