Whenever you check for your confirmation status, if you see it as flushed PNR, it could mean that the number you entered is incorrect, or it could mean that the you are referring to an old and used ticket.
Why flush PNRs?
Since we have a limited number number of digits, there could be a total number of 10 to the power of 10 (ie., 10 billion) numbers. Owing to the fact that there are millions of railway travelers a month, this 10 digit numbers will run out easily, especially in countries in India and US where the population is high. This will enforce the government of India and the ministry of railways to flush the PNR numbers when the same is no longer required to be stored and tracked – flushing is generally done 7 days after the date of journey. Don’t worry, the same number will not be re-issued to another railway passenger for a reasonably long period of time, so your privacy is guaranteed by the Indian Railways.