In the episode haunted, Robin was fighting Slade in his mind. None of the other Titans really understood what was happening, so Raven used her powers to enter his mind and find out why he was Schauspielen so strangely.
In haunted Raven enters Robin's mind and she can see what he sees, feels what he feels and knows what he's thinking. He lets Raven in his mind. Now, Robin wants to get into Raven's to try and save her but Raven knows it's to dangerous so she says, "Robin, Du know me better than anyone, Du should know there are places in my mind Du shouldn't go, where no one should ever go." Von this I'm pretty sure she means she shouldn't even go those places in her mind herself.