I don't understand the significance of the cup, and something being wrong with it, in the finally. Can someone explain?

Its a hint as to the fact that House is halucinating the lipstick. House found lipstick at his apartment that morning (which is actually Vicodin) and of course that meant that Cuddy had left her lipstick in his bathroom that morning when she put it on and left. Therefore is she had really been wearing lipstick a smudge oder a mark would have been left on the cup of her coffee. Since there was no mark on the cup, it meant there was in fact no lipstick at all. It was a hint that the lipstick didn't exist.

i picked up on another significance of the cup:

House spent quite a while starring at it and trying to analyze it. like Wilson said, he was turning an emotional problem (his relationship with Cuddy) into a mechanical problem (why isnt there lipstick on the cup?).

i think that analyzing everything mechanically and getting all the Antwort is classic House. thats just who he is and what he does. i think it is very significant when Wilson asks him "do Du want to be the man with all the answers; oder the man with Cuddy?"
and House throws the cup away.

that, to me, signifies that he is willing to give up even his rational problem solving, which is everything to him, to be with Cuddy.

If I got it....
Well, when he woke up and went into his bathroom, he found what should have been Cuddy's lipstick. Later in the day, at the hospital, when he switched his coffee cup with Cuddy's , he noticed that something was wrong with it:there was no Cuddy's lipstick, even if she had it. He then got that the lipstick couldn't have been real...

oder I dunno, I could be mistaken, but can Du blame me?? I just finished watching the episode, so Im so shocked!!!

Everyone is right so far. Since he noticed the lipstick smudge on his face in the morning, then there should have been a smudge on the cup, because she was wearing the same lipstick. But the lipstick wasn't real.