"Bad Line" is never defined by a book, however, if it reasonable to assume as an informed consumer that the line is in error, then the book may claim it is bad. As stated above, typos, wrong favorites happen frequently. In those situations my advise is to call the book (or on line help), alert of the possible "bad line", if the line manager says its good (they sometimes don't like to admit the error), then bet the line and request a transcript of the conversation (on line) or name of person on phone, noting the time of the call in your records.
What tends to happen as\nd pisses off the bettor is when a book is "slow" to move a line, creating an opportunity for the bettor.
An example was the CBB Michigan/Iowa game. Line opened at around -800/+650 and then began to go down to to -600/+450. But one of my books never moved the line. So I took -600 and scalped it with +650 for a nickel early in the day. About 2 hours after my wager, the book I played +650 called me to say they were cancelling the wager becuase it was a "bad line". I had to jump through a few hopes to show them that the line was "good" early in the day based on conversion of the ATS line and other books "history" of line moves that day (pinny). I finally was abe to prove my point and they let the wager stand, unfortunately they no longer looked at me as a "square" and watched my play more closely. Ultimately they kicked me out.