>A German national can gain entry into the
>U.S. with a Cuban stamp in his/her passport.
If the agent deems to let them in. I used to have several large arabic-laden visas in my passport from Egypt and Jordan. I used to get so much hassle about them that eventually I paid to get a new (Canadian) passport.
However, gaining entry is at the discretion of the border official, who may take umbridge at a stamp s/he doesn't like.
>I'm an American citizen and I have a Cuban stamp in my passport and not >only has that never stopped me from re-entering the country
If you are an American citizen, don't your border agents have to let you in regardless? Where else are you going to go? Live at the border station? They might fine you or whatever, but they have to let you in.