A seer can also be mistaken in some details 

No one can describe the spiritual world in its entirety; it is more complex and vaster than our physical world. I admit that the spiritual researcher can be mistaken in some details, just as one can be mistaken in the physical world, if one wants to determine, for example, the height of a mountain. But no more than such a mistake in detail can be a reason to deny the physical world, no more may the reality of the spiritual realm be rejected because of a flaw in some specific detail.

Source (German): Rudolf Steiner – GA 88 – Über die astrale Welt und das Devachan – Berlin, October 28, 1903 (page 32)

Translated by Nesta Carsten-Krüger