Sstrathern has commented a number of times on da Vinci's notorious habit of starting painting commissions and not completing them. His interests always took him away from the final brush strokes.
I love this footnote. From the main text:
The following year Leonardo started another major painting, The Madonna of the Yarnwinder, but this too would remain unfinished.*
* The original of this work has been lost, but copies have survived. Only the copies are completed paintings.