Your bank's cash machine has gone berserk, giving out 20's as if they were 5's, yet your receipt shows no sign of overpayment.  You immediately know that something is wrong.  You asked for a withdrawal of $100, but it gave you $400.  What do you do?  Would you a) Use the emergency telephone to alert the bank.  b) Take the money and run,  or c) Call your friends and tell them to get back there too, as you go to the end of the line for a second run at the machine yourself?