A Room With a View- What is the "slip" in Lucy's speech?

I just read A Room With a View, and there is a part where Charlotte comes to visit Windy Corner and Lucy reassures her about George:

"“What I mean by subconscious is that Emerson lost his head. I fell into all those violets, and he was silly and surprised. I don’t think we ought to blame him very much. It makes such a difference when you see a person with beautiful things behind him unexpectedly. It really does; it makes an enormous difference, and he lost his head: he doesn’t admire me, or any of that nonsense, one straw. Freddy rather likes him, and has asked him up here on Sunday, so you can judge for yourself. He has improved; he doesn’t always look as if he’s going to burst into tears. He is a clerk in the General Manager’s office at one of the big railways—not a porter! and runs down to his father for weekends. Papa was to do with journalism, but is rheumatic and has retired. There! Now for the garden.” She took hold of her guest by the arm. “Suppose we don’t talk about this silly Italian business any more. We want you to have a nice restful visit at Windy Corner, with no worriting.”

Lucy thought this rather a good speech. The reader may have detected an unfortunate slip in it."

For the life of me I can't figure out what Forster means, even after reading it several times. What on Earth is the "unfortunate slip" that she makes here?