The iTunes Library is hopeless

The iTunes Library file cannot be saved. An unknown error ocurred (-51)

This is a first timer for me... Popped up about an hour ago.
Clicked Ok on it, and continued to listen to my music with out any problem whatsoever.
Closed iTunes, re opened it, no problem at all.

There are a lot of Google pages dealing with this, all of them with different error codes, the most common seems to be the number 50.

Google: The iTunes Library file cannot be saved. An unknown error ocurred (-51)