In particular (and you'd think this would go without saying, really) don't send someone a message starting "Hi [wrong name here]!". It results in red faces for all parties concerned, and most likely the premature termination of any futher contact.
Luckily, in my stupid case, the bloke concerned had a sense of humour about it and messaged me back, making a joke about the conveyor-belt feel of the dating site we are both navigating.
It is so easy to slip up in this strange online world where you can go "cyber-shopping" for a match, scrolling through page after page of eligible men, adding some to your favourites (ie. the trolley) and being added to other people's favourites, all the while trying to manage the varied messages that come your way. A few blokes have sent me one essay-length message after another, back and forth, back and forth until I find my heart sinking at another "ping" in my inbox. Let's meet up already, dudes! There is no point wasting good Facebook time if we meet up and the chemistry is off.
Speaking of which, it makes sense to bone up on your potential candidate (and I do mean "bone up on", not just "bone" - the devil is in the detail) before you meet them, in order to avoid this kind of embarrassing pratfall. Otherwise, just refer to the first rule, and stick to safe topics like global warming, the upcoming Mayoral election, and Conchords vs. Boosh.
It's a potential minefield for the chronically vague. I only hope someone out there will one day benefit from my mistakes.