I'm a little over a month into a 9-month teaching position, to cover two semesters' worth of courses. It's my first "proper" job since finishing my PhD. I've just been offered a 2 1/2-year postdoc (which I'd much rather do than my current job), and been told that the latest I could take it up by is 2/3 the way through next semester. So if I left to take up the postdoc, my current department would have to find someone else for (at least part of) next semester's courses.
According to the terms of my contract, I'm within my rights to leave, giving 3 months' notice. But if I did that, would I piss the department off so much as to find myself in some way blacklisted in years to come?