Most of us have had that funny feeling that you have experienced something before, but can't quite place it.
Until now, scientists have been in the dark about the strange phenomenon known as déjà vu, meaning "already seen" in French, but new evidence suggests that not experiencing it could be a sign of health problems.
Researcher Akira O'Connor, from the University of St Andrews, UK, has been exploring the concept of deja vu, and believes it's a way of the brain checking that its memories are correct.
As part of his study, O'Connor used MRI scans to monitor the brains of 21 volunteers after they were given a sequence of words to trigger memory.
The scans showed activity in the frontal regions of the brain, and signals that suggested the brain was checking through memories and sending messages if there was thought to be some kind of error.
Prior to O'Connor's research, scientists believed déjà vu was the brain making false memories, but this is now believed to be wrong.
"Brain regions associated with memory conflict, rather than false memory, appear to be driving the deja vu experience," O'Connor wrote in his blog.
"This is consistent with our idea of déjà vu as the conscious awareness of a discrepancy in memory signals being corrected."
O'Connor said his findings helped to shed some light on why déjà vu seems to happen less as we age, despite the fact that memory problems happen more often as we get older.
"If it's not an error, but the prevention of an error, this makes a lot more sense, he wrote."
According to New Scientist, a brain that isn't checking itself for memory errors, therefore triggering déjà vu, could be a bad sign.
Although O'Connor said these people who don't experience it may just have better memory systems in the first place.