When a capped cursor is overrun, the following warning is displayed on the server:

warning: collection dropped during yield of collscan or state deleted

Perhaps it's not worth warning in the case of an overrun, given that this is the nature of a capped collection (and the client can interpret an empty result set with a dead cursor as a possible indicator of this state). If it is worth logging something, it would be helpful to explicitly state why the cursor was invalidated.