heck if you have a conic volcanic structure the drainage pattern is a radial drainage pattern.... but eventually the radial streams will coalesce into one drainage feature. Rivers flow downhill regardless of the direction downhill. The big "no-no" that is consistently viewed with rivers is the branching and eventually reconnecting haphazardly, and that doesn't happen in nature except in river deltas and the braided stream areas in a glacial outwash plain. The second worse violation is rivers flowing past each other into different large bodies of water... there has to be some kind of continental divide and drainage basin setup going on with drainage patterns, since all water flows downhill rivers shouldn't flow past each other without some major elevation change in between them.

Have I forgotten any major river police rules?