Or questions that are original but get marked as duplicate. This has happened to me several times. It's become my biggest frustration with StackOverflow. I might run into a problem that is a new variation on an old problem. I try to document it carefully, and I often link to old questions on StackOverflow so that people will realize that I've done my research, and my current problem is not the same as the older problem. But still, the moderators on StackOverflow don't always take the time to read all the way through a post. They seem to just read the first sentence or two and decide that the question is a repeat of an older question. I've been shocked at how aggressive they can be about marking a question as a duplicate.
Another frustration is stuff like PowerShell where it's entirely luck of the draw whether they answer your question or redirect it to SuperUser (where nobody is likely to be able to answer certain classes of question anyway).
