Virtually every shop out there addresses the "bad line" issue in their rules. Here are a few examples from some top-rated books:
BETJAMAICA
From: http://www.betjm.com/asi/WPHRules.asp
BetJamaica reserves the right to refuse or limit any wager. This may include, but not limited to past posted plays, obvious line errors, or exceeded bet limit amounts.
WSEX
From: http://www.wsex.com/about/wse_rules.html
When a line is posted in obvious error due to a typographical mistake by the book, any customer wagers entered to take advantage of the error will be cancelled. Note: “Obvious errors” include cases in which the wrong team is posted as the favorite or in which the number posted for a point spread, total, or money line differs significantly from the correct figure.
PINNACLE
From: http://www.pinnaclesports.com/Rules
Pinnacle Sports reserves the right to cancel any wager made on an obviously "bad" line or a wager made after an event has started.