Not specifically seen it on the Silmarillion, but have seen it occasionally on other books, including a Morgoth's ring 1st. It is one of those random cock-ups that seems to happen for time to time. For most books, it seems to render them worthless (not necessarily uninteresting, though!), unless the error denotes some specific early state.
Now you just need to find some rare postage stamp where part of it is printed upside down, and you might get rich (although from what I understand, stamp collecting is in a bit of a lull)!