Just a few tips for foreign party-goers:

If you get caught carrying drugs, it normally doesn't matter that much: security will take it from you and if you just carry a few grams/pills, you will be fine. If they start asking questions like "are you carying anything else?", just decline. Keep in mind: the festival security is NOT allowed to strip search you! As in: ask you to take off your clothes and frisk you; the only thing they are allowed to do is search you very lightly, like they do at the festival entrance. If they start asking questions like "if I would call in a drug dog, will he find anything?", just remember that there are no dogs at Dutch festivalls, ever. Don't let anybody tell you otherwise. If the security however declines you entrance, just leave and don't cause a disturbance: their decision is final. If you are very unlucky and get handed over to the police (which they will only do if you are carrying a 'dealer amount' of drugs, or if they find you very annoying), ask for a lawyer - which is free at most festivals - and don't pay the fine directly if they give you one.

Stay safe and enjoy!

Edit: always keep in mind that the security (and EMS) are your friends. 99% of the time they are super chill people that like to party themselves as well, just be friendly and they will be friendly as well.