well the dumbest thing i have ever done is coughed into a bowl or dump it out before its finished. I have a friend though who ahs been smoking for years and he fucks up the rotation so badly its not even funny. He like wont pass it or hell take lke 3-5 hits instead of 2. People screw him over badly and he constantly talks about how high he is even when hell share a joint with another person and thats all hes smoked.