Kind of hard to describe - basically if you place an outpost in an area far from any of your cities/units, where you wouldn't normally have line of sight, and the zoc of the outpost encompasses multiple resources, building on one of those resources 'dims' your view of the other resources, and for a moment you can't select them.

Clicking off the resource you just built and selecting the outpost or another unit seems to fix this quirk, but it causes repeated clicks on other resources in the zoc to do nothing at first, which is pretty annoying.