My son was invited to a Swiss birthday party. This really doesn't happen all that much to my kids here, in comparison to the international school in Warsaw where usually the entire class was invited. But anyways, in The Netherlands when you go to a party you also congratulate the parents (and grandparents and brothers and sisters and other visitors...), so I immediately started kissing the parents on the cheeks (yes, three times) and said "congratulations!" very cheerfully. Then came an awkward moment and the dad stumbled....."you know it's not my birthday but my son's". Err-mm, yes, I know....whoops!
Also, in Poland when the kids at our birthday parties even smelled candy or cake they would run up to the table putting sweets in their mouths. A little bit like the opening of the birthday presents, quickly and if possible at the door in The Netherlands. The first time we had a birthday party for our daughter in Switzerland, all the girls nicely sat around the table waiting. I thought maybe they didn't like the cake, but in fact they were waiting for my signal to start eating. Wow, that never happened to me before!
So, be aware and don't automatically assume that what you are used to is also a custom in another country.