Be mean, rude, or disrespectful to any of the staff. This is a big one. We understand everyone and their co-founders wants a meeting with Dave, but you won't get one by throwing a tantrum or being threatening to his EA. If you demonstrate a sense of entitlement and be a jerk, it will almost certainly keep you out. The partners at 500 cares a lot about company culture and keeping the 500 family awesome, so show that you have not only a cool startup, but that you're a cool person as well.
Be extremely verbose with your pitch. More words are not necessarily better. Everyone is busy so keep it short and focus on key points. No need to write a novel on how your dog died so you stayed up building a dog memorial site in php but the UI was terrible you had to scrap it and find a designer on Craigslist but instead found your long lost cousin who turns out to be a ruby on rails developer who once high-fived Eric Ries and loves french toast blah blah blah 20 pages on the psychology of dog owners..a picture of your distant aunt in Texas..you get my point. Anything too long will immediately make the partners and mentors want to skip it. Be concise and focus on the problem, your team, notable metrics, and your idea.
Just "show up". Sorry we cannot encourage this due to security reasons. We appreciate your enthusiasm but we can't just let you enter our office unannounced to network and pitch. The office is locked anyway. There are other ways. We have a big network--getting connected with someone in the 500 family is a much better approach.
Be outrageous, crazy, and entertaining but forgetting to focus on what's most important, your startup. We'll appreciate you for giving us laughs but we'll still look for key characteristics of a strong startup. If you're very entertaining but you failed to convey your startup's strengths, your potential, your capability of running a great business and grow it, then we cannot consider investing in you. After all, we are an investment fund.
This is all that I can think of right now..I may revisit add or edit. :)