Not much trouble slipping into Palestine for a foreigner, but you do get interrogated on your way out of Israel right at the airport and if you say you went to the occupied territories you may get sent to the little room for further questioning, search or seizure of your belongings, notebooks, laptop, phone, etc. You might also get put on a list for secondary screening or outright ban if you want to go back. So think it though if you want to go to Palestine -- it is indeed considered a theatre of military operations by the occupying power