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yes, google will penalize you for that!
I've often wondered if G would get annoyed with me, because I do have a habit of changing titles constantly. So far I'm not aware of being penalised for it. (But I shall be more wary in future.)
However I should say that pages that I change are usually heavily linked to and I'm fairly sure that changes in the title aren't going to change the position in the SERPs.
I'm always changing the title to my home page, for example. That page appears in most of my target keyword searches despite never having had my target keywords in the title. Google seems to have figured out what my site is about and now couldn't care less what I put in the title. That is fine with me.
I think there are legitimate reasons for rapidly changing titles aren't there? Like rapidly changing, or updated content? Blogs come to mind.
Seriously unless you're a news site,... there's no point.
Even if Google takes note of the new title within a day ( it just happened on one of my sites, same-day indexing, caching, listing, whatnot ) it won't rank the page for the newly added words until the next overall relevance calculation, which takes at least 4 days. But as it doesn't occur every day, it's more like a week.
So again, apart of an obvious red flag, and ranking for stuff you don't have anymore, I don't see what could happen.