3. Being Too Familiar
It's always a good idea to connect with people at other companies, but don't treat them like your close friends who understand or appreciate your sense of humor. Remember that these are people you're trying to impress with your professionalism and knowledge, not with your quirky-but-fun-loving habits such as sharing the world's largest collection of fart jokes.
Avoid email forwards or referring people to funny or semi-inappropriate humorous websites. The rule of thumb is that if you have to label something NSFW (Not Safe For Work), it is Not Safe To Forward. Better yet, just don't refer anything funny or of a personal nature to people you're trying to work for.