The mind is a powerful thing, and all we are is based on our minds and what is fed to them through our senses. If you believe something strongly, perhaps strongly enough that your senses create tangible things that support that belief, then how is it not real in every way that matter to you?
Now, if that belief is that you dun goofed when summoning a prince of hell or some shit and you're about to be ass rammed (none to gently, either) as penance before being cast to the sixth circle for your goof, that may trigger some pretty serious panic I'd imagine.