Actually, as unexpected goes, this is hardly the most bizarre unexpected Jonathan Strange has encountered in his years as a magician. It does not even rate the top hundred list, as it happens. Jonathan Strange's life lately has been... well... strange.
Therefore, walking through a door which previously was known to lead to the library, and discovering that it now leads, instead, to what appears to be a large assembly room of some sort, designed in a very elegant, if curiously unfashionable, style, does not in the slightest take Strange by surprise. Considering that he was previously in the house of a magician (and doors which one believe lead to Norrell's library do have a reputation for leading elsewhere, after all), he hardly even notices it.
"Oh, damn," he says, stopping short just within the door. "This is certainly not where I wanted to be." He turns to leave just as swiftly as he came, but upon opening the door, he discovers that it now leads to a grand, sweeping lawn, beyond which the green and grey smudge of a forest can be seen, and not anything like a corridor of Hurtfew Abbey. That is hardly the only thing that is different, however...
Strange looks up at the cloudless, warm blue sky; he looks slowly round at the sun-drenched lawn, and the wrought-iron rail of the front porch, which glitters in the sunlight. He steps back inside, a bemused smile coming slowly into existence upon his face.
"I am not quite sure how I managed it," he says aloud, "but I do believe I have finally succeeded."
Typist: I know, took me long enough. x____x anyway, Jonathan Strange, from the novel Jonathan Strange & Mr Norrell, by Susanna Clarke. And, to anyone responding who doesn't know the book - Strange is from the early 19th century and dressed accordingly.