I had a problem when Oracle used mounts that were not explicitly listed in exports. For example, I had mounted /vol/vol1/orafiles to /orafiles, but in the exports file on the filer I only had listed /vol/vol1. Everything was fine for weeks until I failed over. At that point, it was still mounted but it wasn't really there(permission denied, other weird messages) even though other similar mounts were fine and usable.

I have since gone back and exported every directory that I mount to and I have failed over (and even survived some core dumps) without any further problems.