I've noticed that there are a lot of postdocs postings that offer fixed-term contracts for 12 months and sometimes less (I did find 8 months). I'm looking for a postdoc position as my first post-PhD job and I don't know if I should also consider those. For the short-term contract, what's the usual story? do they really expect you to publish in just 8 months?
Typically very short (<1yr) positions are offered because the PI only has money to pay for a few months' postdoc time. This can arise for a variety of reasons, but the most common is probably that they obtained a grant to pay for (say) a 3-year postdoc, and then hired someone who found a new job after two-and-a-bit years. This is an awkward situation: often funding agencies place strict time limits for money to be spent, so one may be faced with the challenge of getting rid of a substantial pile of cash quickly, and hiring someone is the only realistic way to do this.
Related, but distinct: sometimes PIs will choose to spend the dregs of a grant on employing a graduating PhD student for a few months after they complete their thesis. The length of position offered is usually directly governed by the amount of money left in the bank. In such cases it may be necessary to 'advertise' the job to satisfy employment regulations and/or policies.
Unless you are already involved in a project, it is difficult to achieve much in a postdoc that lasts less than a year. Inevitably you lose time at the start getting settled in (especially if you have to move to a new city), and you will be busy applying for new jobs and attending interviews. PIs' expectations no doubt differ, but I think it is unreasonable to expect much to come out of such a short position.
Often PIs in this situation will be applying for follow-on funding, so there is a chance that the position will ultimately be extended to a decent length. However, you should not count on this happening.
If you see an advert of this sort, I think it is reasonable to contact the PI and ask for an explanation of the circumstances, and how they envisage things working. Personally, I would only apply for such positions if the alternative were unemployment.