I was reading Unfinished Tales, and I was reading the part about the Istari and came across this.
The essay on the Istari just cited thus tells much about them and their origin that does not appear in The Lord of the Rings (and also contains some incidental remarks of great interest about the Valar, their continuing concern for Middle-earth, and their recognition of an ancient error, which can not be discussed here)
To what exactly is this ancient error referring to?
Was it letting Sauron stay in Middle-earth after the War of Wrath?
Not defeating Melkor a lot sooner than they did, which allowed his evil to spread?
Or not helping heal Middle-earth after the War of Wrath?