I've been reading a lot of early Holmes such as The Sign of Four and the Adventures/Memoirs and here are some things I have learned from these great Doyle stories (other than "when you have eliminated the impossible, whatever remains, however improbable, must be the truth" of course- I already knew that from Study in Scarlet)
-I've learned to fear receiving five dried orange seeds in the mail.
-Significant conversations are often had on trains.
-The likeliest murderer is never the murderer.
-Disguises are best used often and are extremely effective.
-War wounds tend to migrate.
-You can judge a man by his pipe.
-People often carry a large collection of significant items in their coat pockets at the time of their death
-In Watson's timeline, things often happen out of order (time traveler I have to assume)
-The motives of women are inscrutable.
And finally: Sherlock Holmes is not always right (!) "If it should ever strike you that I am getting a little overconfident in my powers, or giving less pains to a case than it deserves, kindly whisper 'Norbury' in my ear, and I shall be infinitely obliged to you."- The Yellow Face
It was inevitable.