This comes up several times a year. Maybe Cliff Lee has a nagging injury and is 50% to make his start at home the Pirates. Someone posts "Hey Cliff Lee might not start, I suggest betting Pirates +250 with action! If they have to substitute Kyle Kendrick or some scrub the line will drop like a rock! And then you can arb it with Phillies -140! So you have a 50% chance at a huge arb!" Lots of people reply saying "Great suggestion! I've bet the Pirates!" Then lots of other people reply "That's not how action works. The line is adjusted after the pitching change." Invariably the person who suggested it says "That's not how it works at my book, my book locks in the line when you bet action!" And when asked, the person mysteriously isn't able to name the book that doesn't adjust the line.

The simple fact is that a book would get killed if they didn't adjust for price.