I fetched a forked mysql driver and got rid of this error for strings
fetched from the database.
BUT all I had to do to see this message again was to enter a non-ASCII
character in a (login) form field and there it was!
It doesn't matter that I don't even allow non-ASCII characters right
now because there's a validation regex restricting to [a-Z0-9].
That means any user can disable my application with the innocent usage
of a perfectly legal character.
Last time I inquired about this issue was last September, and after
hearing "no solution" I abandoned any attempts of pre-beta Rails 3
testing. This is so essential, and so easy to trigger! - I wonder why
it's still so easy to cause a major disaster?
Oh, btw, using the latest git/svn versions (as of this minute) of
Rails3-master and Ruby 1.9.2-head.