It's hard not to judge a whole group by the actions of some of its members.
I'm thinking of somewhere like Magaluf, where as I understand it, the young Brits rampage about horribly drunk all night having indiscriminate sex, making a noise and so on. The Majorcans could be forgiven for imagining that British people have few morals and spend most of their time inebriated.

Similarly, we seem to have quite a few beggars in Norwich of Romanian origin (I have asked them while offering a bit of money) It would be easy to assume that Romania is populated entirely by indigent scroungers.

I find myself subconsciously making generalisations like this, and try very hard to stop doing it.