I stopped at page: 15
Why I stopped reading: Historical implausibility. It's really as simple as I couldn't believe a major part of the character's back story. But why would that just make me stop? Because it makes me wonder -- Does the author really know this time period well enough to write in it?
The lesson to be learned: To illustrate the moral of this week's story, I want to refer back to experience I had while writing my regency novella A CONTEMPTIBLE AFFECTION. At the end I really, really wanted my heroine to plant another lady a facer. A few beta readers thought it was funny, but when I consulted someone I really trusted about regency etiquette and rules, she told me that it just wouldn't happen. A lady of quality would never use her fists, but rather her words. The resulting edit turned out WAY better. So know your stuff.
And not everything has to be absolutely believable. Georgette Heyer's THE GRAND SOPHY (my review here) is ridiculously unbelievable, but within believable bounds, so it works. Just don't go too far. Or if you plan on going too far, make sure, like Sophy, the character is so "in character" that it works out.
Writing historical novels is hard. It takes A LOT of research. In the best books, the success is all in those little details the reader doesn't even realize you spent three hours digging through history to find -- the ones that don't sound like history lessons, or worse, history lessons gone wrong. :)