Bastek = Sebatian, Basha = Barbara
these examples are funny :)
Sebastian and Barbara are official names, in Polish, Basia is a very popular diminuitive (Bastek isn't), but when I introduce myself to a stranger I use the formal name, as most people do in Poland, diminuitive forms are mostly used by family, friends or people who know you at least a bit.
And I have no problem with my name, but foreigners do :)