After being made knight by Galahad, Sir Melias rode into an old forest, where he found a lodge of Boughs. Inside this lodge, on a chair, was a subtly wrought crown of gold. Melias took the crown and almost at once was accosted by another knight, who fought him for taking what was not his, won back the crown, and left Melias wounded until Galahad came and found him again. This happens during the Grail adventures.
The crown symbolizes wordly pride and covetousness, and may be an illusory temptation rather than an actual physical object.