Cassandra's Tears is a prophetic condition, in which the carrier falls in a seizure and has visions of the future. It is first mentioned in Grave Peril.
The only person mentioned in the series with Cassandra's Tears is Lydia, who has had three visions of a spirit coming for her. She asks Dresden for a talisman, something for protection against the spirit.
According to Harry Dresden, the person who has Cassandra's Tears has random seizures while having visions of the future. They are always vague and hard to believe. Doctors mistake it for epilepsy in children. The prophecy is usually pretty accurate but no one buys into it. It's called a gift, but most often is consider a curse.
Cassandra's Tears is often a used as a scam and is common in magic circles. The claim can not be proven. They only would need a little talent to have the right aura and the ability to act.