Ah, man, I even did a (apparently poor!) search before posting - thanks for the merge.
You can always take a look at the list of the confirmed bugs .
But it's good to get reports for such stuff after patches, so that we can see if they've been fixed or not.
Edit: from some of these examples, this appears to happen when a river has two 'mouths.' I'll have to look for a river with two sources, though I think I recall those are ok
mmhh...maybe someone can reproduce it with the worldbuilder.