Bad line means 50 cents off from other conventional books generally, or the pointspreads between dogs/favorites reversed / significantly different.
Betting obvious bad lines is idiotic. I generally tell the book immediately when I see they graded a wager wrong / have an obvious bad line out.
Another one to add is the idiots who get a wager marked as a win by accident, so they try to withdrawl everything immediately / place a new wager with funds they should not have had.
Individuals who deposit with creditcard and then issue a chargeback too, although I guess that would qualify as a stiff.