Since there is no safe inserting going on mongorestore should throw some warnings up.

If --drop is not used, it should warn if the collection exists.
"Restoring to 'coll-name' without dropping. Restored data will be inserted without raising errors; check your server logs"

If restoring against a non-master/read-only
"You are attempting to report to 'coll-name' but you can't; +<reason>"

These address some of the default assumptions of many. 1) It will complain if there are problems, 2) it works if there are no errors. There are probably many other cases where this is not true, but these are two obvious ones.