I heard this from two different people in two different situations during my stay here in Poland. I actually didn't ask any of them about it. They told me alone on several occasions. These people don't know each other either and both said they know Americans who do this all the time. I have no reason to believe that they would mislead me, but I am curious where exactly I can find more information.
Does anyone know exactly what he is referring to?
And what could be the consequence if you then travel to other Schengen countries?