Quote Originally Posted by kalinka_vinnie
I am sitting in an armchair - OK
I am sitting in a chair - Error

I have also never heard of "sitting in a bench", but I have heard "sitting in the couch"... or have I?
I think you can say both "I am sitting in a chair" and "I am sitting on a chair".

I don't know about "I am sitting in the couch". It does not sound right.

"I am sitting in a bench" definitely sounds weird to me.