I recently made a post about a biblical contradiction that wasn’t necessarily the most damning of contradictions. The post on the issue of demoniacs and geography laid out these two issues:
We have two problems here:
Problem 1 – the Gospels contradict each other on where this took place – the country of Gadara or the country of Geresa
Problem 2 – one or two demons
Most skeptics claim that there is clear contradiction in both of these. The accounts are clearly referring to one single event, so there can’t be two events in nearby places.
I was more interested, in that post, in how the Christian mind deals with such contradictions rather than the importance of that particular contradiction. An arguably understandable comment came in from See Noevo (he quotes me first):
“I had been discussing the contradictions of the Bible. As mentioned, many are fairly irrelevant in the scheme of things and don’t really invalidate the core claims of the Bible, only the claims of inerrancy.”
But what if the supposed lack of inerrancy is irrelevant in the scheme of things and doesn’t really invalidate the core claims of the Bible?
I didn’t read past your quote of Mark 8:26-27, but how is the scheme, or theme, of Jesus expelling demons invalidated by the exact number of possessed people or his/their exact geographical location?
The problem for the Christian who claims this, however, ends up being more serious than this prima facie approach. What happens when one pulls a thread is that things begin to unravel. Like a garment, you might see a single thread and think nothing of it unravelling. After all, compared to the garment as a whole, it is insignificant. But, often, that thread can start a larger process of ruination, detracting from the perfection of the piece of clothing. In reality, with the Bible, there are many such threads, too. It is not just one single fine thread. Some are entire sections. As I mentioned in previous posts, my book on the nativity accounts (The Nativity: A Critical Examination) shows that the entire birth narratives of God incarnate are incoherent. 
To move away from the analogy, though, we have a deeper epistemological issue (and one that serves as one of my main conclusions in the aforementioned book). If there are a number of insignificant contradictions, in one sense (for the liberal Christian), it matters very little. But in another sense, such contradictions have huge ramifications. What they do is weaken the certitude one might have that the Bible is reliable. They sow seeds of doubt that any claim made in the Bible is verifiably factual. When you start picking holes and people say, “Well, that claim is not very important, so it doesn’t matter”, how can you know that more important claims ARE factual?
If you conclude that Mark or Matthew got some minor detail wrong, then you are raising the probability that there is some other more significant factual error. You enter Bayesian territory for analysing probabilities and increasing the prior probability of finding issues of veracity in any other given passage. You may never know what these errors are because they might just happen to be unverifiable. But, just in a general sense, you are undermining ideas of reliability within the Gospels.
You could take this one step further in analysing how this might work with OmniGod at the helm.
Why would God allow his inspired word to be littered with errors that we are unsure don’t expand into more serious claims?
God is already in trouble with supposedly knowing the future, yet still allowing his revelations and inspired Holy Book to be used to countenance slavery (Curse of Ham, anyone?) for almost 2000 years. When he allows outright falsehoods to get past his proofreading checks, then serious concerns must be raised.
It’s why I think liberal Christianity is often much more difficult (despite the preference of its adherents to the atheist) to uphold. Liberals have to cherry pick and harmonise on a constant basis, and that mental gerrymandering takes its toll. The presuppositional literalist just says, “sod the lot of you, it’s all perfect and right and there’s nothing you can say that will convince me otherwise”.
Therefore, despite the above comment, I think small contradictions in the Bible are more serious than some people think.