Google refers to them as "rolling" tests. They randomly select query strings from a given demographic group; then, from another group, etc..
This is what I've been seeing for a long time now:
"Showing results for multifunction machines. Search instead for mulitfunction machines"
By default, the correct spelling's results are shown. (That's a very common typo, not misspelling, but typo due to the way our brains and fingers get transposed for those that know how to type. Those that type know how it's spelled, but the fingers just won't cooperate).
But clicking the link for the typo:
You'll see how common it is and you'll still see sites ranking for the typo-error (misspelled) word.
But then you get those annoying occurrences where Google thinks the wrong spelling is the right spelling. I was just searching for "backmans island mahone bay" (which is in Nova Scotia if you need to be more specific) and its says "showing results for bachmans island nova scotia". This then did not bring up the link I was looking for (http://www.mahoneislands.ns.ca/event...cnic2011.shtml).
I assume this happens because more people misspell it than get it right.
Dynamic Software Development
During keyword research, I do encounter odd typos that actually rank as keywords. From that, we can derive two things. One, typos aren't all that bad because they can be additional long-tail keywords. Two, Google is definitely strict with proper spelling, but we already know that, given that pluralizing a word with "s" at the end counts as hitting a new keyword.
They are also doing something incredibly annoying and rather insane for site owners: I have to search for a lot of manuals, and in searches for: [product name] manual, they are hitting on and bolding in the results: instructions, userguide, user guide, guide, etc. Yesterday I searched for a product along with: irregular heartbeats, and arrhythmia, and for each search they were returning the other in the results. (But for some odd reason I can't duplicate that right now! )
I guess that sort of thing makes sense for some people, but it should be an option, and not mandatory! There should be a way to turn that OFF. It's ridiculous to now have to put every frickin' word in quotes in order for that word to show in the results! (And even then they still return irrelevant pages).
This is merely a change that relies on a condition. If you are in an industry where misspellings are the norm (like in medicine), then you should spend all the effort you need to fix it in your site. If you are in something that's about widgets and only contains common words, I doubt the misspellings will make a difference since it is highly unlikely that someone will find your site by typing a common phrase. Correct me if I'm wrong, but it only penalizes you if people actually go to your results...