On page 305 of Unfinished Tales, there is a passage regarding the boundaries of Rohan:
The bounds of the realm of Eorl were to be: in the West the river Angren from its junction with the Adorn and thence northwards to the outer fences of Angrenost, and thence westwards and northwards along the eaves of Fangorn Forest to the river Limlight; and that river was its northern boundary, for the land beyond had never been claimed by Gondor.
Here, it is stated that the western boundaries of Rohan extend from Isengard 'westwards and northwards along the eaves of Fangorn Forest to the river Limlight'. How can this be true, given that Fangorn is to the east of Isengard?