He might come, but I should be surprised.

If what you are trying to communicate is that someone might come to a meeting or party, but that you were told it would be better for some reason for you to act surprised if he does appear, then the sentence doesn't make any sense. A better choice:

He might come, but I should act surprised if he does.

I'm not so sure it's grammatically correct.

The word "should" should be "would".

"He might come, but I would be surprised if he does."

No, because the writer is already aware that the man might come. So the writer cannot be actually surprised if the man shows up. This indicates to me that the writer of the sentence has some reason to believe it would be better if he acted surprised if the man does show up.

I get a different context. Surprised meaning figuratively as in not really expecting him to actually show up.

Possibly you are right. However, Tanguatlay will have to explain to us what exactly he is trying to communicate. I said in my first post, "If what you are trying to communicate is that someone might come to a meeting or party, but that you were told it would be better for some reason for you to act surprised if he does appear....", then my sentence would be the better choice. If Tanguatlay is trying to communicate what you are saying, then your sentence would be better.

I wonder if Tang means:

He might come or not. I would be surprised if he did.

or

He might come or not, however I would be surprised if he did.

My daughter said one of her classmates wrote the sentence and asked her whether it is correct. My daughter asked me if it makes sense. I had no answer, which is why I posted the sentence. And I doubt her classmate knows what she wanted to say.