I love "ER" (even though I won't touch the whole Carby vs. other Carter pairings and Abby pairings feud with a 39 1/2 foot pole). One of the things I like the most about it is the various friendships between the doctors, nurses, med students, whathaveyou in the hospital. Which is why I can't understand why, in almost every fic I read where Doug and Carol come back to Chicago from Washington that is set after season 8, Doug and Carol have no idea that Mark died until someone in the ER tells them.
I know that at Mark's funeral Doug and Carol were no where to be seen since George Clooney and Juliana Marguiles were unavailable for the taping. But there has got to be a better explanation for those two characters not to be there than "they didn't know Mark was sick/was dying." Mark and Doug were best friends for God knows how long. Mark and Carol, too, especially after Doug left Chicago. Hell, Mark was there helping Carol through the birth of the twins, and Doug owes any future children he has with Carol to him. Why would Mark leave them out of the loop about his tumor?
There are several valid reasons that could be used to explain Doug and Carol's absence at Mark's funeral. Someone in the family could have been sick (or Carol pregnant again) and unable to travel. Perhaps Mark and Rachel stopped by Seattle on the way to Hawaii to pay them a visit and Doug and Carol said their goodbyes there, and didn't want to go to Chicago for the funeral. Or else they couldn't get time off of work (lame, I know). But the excuse of Doug and Carol not knowing that one of their closest friends had died is just unacceptable.