arrested? they must've used a lot of fake $50 bills at once? otherwise any innocent dude can end up with a fake bill unintentionally...
once a mcdonald's cashier told me that my $10 bill was fake... which was not, it was an older edition that didnt have the line of silver markings. i think cashiers should be trained to handle these situations better.
speaking of which, when my family exchanged some yuan for canadian dollars in Hongkong back in 2004, the HSBC there gave us $1000 bills. When we tried to deposit them at our bank, those tellers had to search for their ultimate canadian bill book to check and make sure those bills were legit. apparently they had never seen those bills before