A couple of gestures that can lead to problems...
Holding your hand up with the palm facing the person can be extremely offensive to some Greek people (very easy to forget that one when explaining to a non-English speaker when your are trying to tell them their flight leaves from gate 5 !).
Joining the thumb and the index finger in a circular fashion - most western cultures will accept this gesture as meaning "OK", the Japanese can perceive it as meaning "expensive", some Arab cultures think it means "I will kill you", and us Brits can use it as an offensive gesture .... be careful how you use it !!!!!!!