While the "S3E21" tag most accurately fits CSI: NY , the title "Past Imperfect" is common in other series:

In this episode, Mac Taylor pursues a serial killer whose conviction is overturned after the arresting officer is jailed.

Stella Bonasera receives her HIV test results, which come back negative. Other Possible "Past Imperfect" Guides

While not titled "Past Imperfect," this specific season and episode number features the crew meeting their own descendants from a divergent past.

Xena has flashes of her past life with Borias while trying to protect a village from an army.