We were grabbing a snack at Sommerfest in the German pavilion on our last trip. My husband and daughter were in line for pretzels and brats, and I went to grab a table. I was just steps from the table when this mom directed her kids to run to get the table. They pushed ahead of me and plopped down in the seats. Mind you, they came from behind me and could see I was headed to the table. I just stood there in front of the table and said, "That didn't just happen." I was floored. For the record, they were American (so am I).
I couldn't believe she deliberately directed her kids to swipe the table. Of course, it wasn't "MY" table, but it was clear she knew I was going to sit at the table and she wanted to prevent that. It was the last table, so we stood and ate our food, while the mom carefully avoided eye contact with us. To make matters worse, they were way back in the line, so they didn't even get their food until after we were done eating. We could have been done with our food and handed the table over to them and everything would have worked out fine for both families if she hadn't been such a jerk.
We have also encountered agressive tour groups. I think their behavior has less to do with their nationality and more to do with their relative lack of adult supervision. My daughter is exceptionally well-behaved when she's with me, but I have no illusions about how she'd behave with 20 other little girls and little to no adult supervision.
I also encountered two young ladies (Americans as well) employing the "my family's up ahead" gambit. This was in the line for Tink, so it was plenty annoying. I let them pass, but we discussed our skepticism with the family next to us for the remainder of our wait. If this happens again, I will tell them that they can go find their family if accompanied by a CM. Otherwise, no.
I have also seen a young (American) lady badgering a CM to let her go to the front of a line because "it's my birthday". To my disgust, it worked. Maybe it was because she pointed to her birthday button?

She was tipsy and giggly, but the smirk on her face seemed to indicate she knew she was getting away with something. Blech.