I was unhappy at my old job so I conducted a brief job search in which I got a very good offer quickly (and therefore didn't pursue other opportunities).
Shortly after leaving the old job (but a few weeks before the new), I discovered information about the new job that materially changes the nature of the position; had I been aware of this I would not have applied at all. I'm now in the awkward position of either accepting a position I don't want or applying to other positions with the "unearned disadvantage" of being unemployed.
Under the assumption that I turn down the offer and look for work elsewhere, how should I explain this to prospective employers? Should I forestall negative assumptions by bringing it up myself, and if so, when?