Re: When a sportsbook offers an obviously mistaken line....
 

Here's what I do:

In Vegas: just bet it. You have rights, there is some consumer protection.

Online: don't bet it. Email them instead. They can and will screw you - either by canceling the wager right away (but jotting down your not a square), canceling the wager after knowing it's a winner (but making it count if you lost), or simply just noting you are sharp, and maybe kicking you out sooner than they would have otherwise.

If you email them about your mistake, maybe you will get a freebie. This has happened to me in the past - I've received free plays for telling them about an error. Knowing that they'd just cancel the wager if I did bet it, getting a free play was worth the email.