I think one of the most common mistakes people make on this type of shot is not being precise enough when pocketing the 1 ball. The 1 is very close to the hole which makes for a really big pocket, and it's easy to just get up there and wack it, knowing that it's going to drop. The problem is that if you catch the 1 a little thinner than you had planned on, or, as is more likely with inside english, you catch it a little thicker than you had planned, then your cueball's angle and speed may change just enough to make you miss a 12" window.