Check out Mike Brotherton’s list of Ten Issues for Hard Science Fiction, complete with handy links.
There are a number of issues that continue to keep coming up in hard science fiction, or any science fiction trying to get the facts right. I just helped my collaborator here a few days ago answer a reporter’s questions on one of these (humans expelled into space without space suits). These things should always be right. There’s no excuse in this day and age. We’ll start with the space expulsion.