I think I figured out what Lily’s blunder, which was actually a betrayal, was.
After the Ancient Kingdom was defeated, Joyboy managed to escape to Wano. Here he met the Kozuki stonemasons and learnt of their ancient language and unbreakable stones.
Joyboy decided to carve his story and the story of the void century into these stones to take with him on his final voyage around the world where he would drop them off to islands that were allied with him.
The World Government got wind of this and knew they immediately had to stop him. They sent Nefertari Lily with the ancient weapon Pluton to go to Wano, destroy it, kill Joyboy and retrieve these stones.
Here in Wano she ended up meeting Joyboy and the Kozuki’s and learnt the truth of the D clan and ultimately turned against the World Government.
Not wanting the World Government to get their hands on Pluton, she buried it deep under Wano and took the Poneglyph of it’s location back to her home in Arabasta to be stored secretly under their temple.
She refused to move to Mary Geoise so the Nefertari family could stay in Arabasta and continue protecting and hiding this Poneglyph throughout history. She left a letter to her descendants stressing the importance of protecting this Poneglyph and keeping it a secret from the World Government.
Lily returned to the World Government and said she was defeated and Pluton was lost. This is “blunder” that Imu talks about in 1085. Of course, it was actually a betrayal, and when Imu heard of the letter and confirmed she was a D, it confirmed to him that it was a betrayal.
Theory by Certain_Guitar6109 (https://www.reddit.com/user/Certain_Guitar6109)