For me, it's better to re-frame the question. How about asking "Why might the price be wrong?" In years of searching I have a long list of possible reasons, and an unpublished book explaining them all. I think when I come across examples I'll share them here.
Garynella was a very hot horse yesterday for the big race of the day at Newbury. When I priced the race up in the week - in a rough and ready way granted - I put him in at a tentative 9/1. Tentative, because he did look like just the sort of lurker the Pipe yard might unleash in such a race, so close to Cheltenham. The big firms probably priced him a bit shorter when they compiled their tissues, say 7/1. But, here's the crucial thing. I price to what I think a horse's chances are. Bookmakers price to that but then have to add other elements. They have to think about what people are likely to want to back, what prices their competitors are likely to offer, and their profit margins.
There was every chance Garynella was going to be a hot horse. There'd probably been some money in the week for him, and some commentators in the big papers tipping him up. So from a bookie's perspective, let's knock a point or two off, and then knock another point off to make sure we're not stand-out price come the morning of the race. And so it happens that, early Saturday morning, Garynella is a best-priced 5/1 shot. And the Pricewise pick. At which point weight of money and fear pushes his price down to 3/1. That's 3/1 about a horse with a chance more likely to have been somewhere between 5/1 and 9/1. A horse being given a 25% chance as opposed to a more realistic, say, 12.5% chance.
Of course, we don't know for sure. He could have won on the bridle. But here's the key thing for me. Here's why the price might have been wrong. The "IF" was already in the price. On Friday evening, the bookmakers had already protected themselves to a point where they were happy. Indeed, come race time the bookmakers in the ring, without previous exposure, were trying to stand him for all they could get at 3/1.
That price surely meant there was "value" to be had elsewhere. Shame I didn't find it! Or, if I did, didn't profit from it.