User reports writing:

Mongo mongo = new Mongo("127.0.0.1", 27017);
mongo.getMongoOptions().connectionsPerHost = 100;
mongo.getMongoOptions().threadsAllowedToBlockForConnectionMultiplier = 5;

... and then spinning up 100 threads, getting "out of semaphores" error. If options are set before constructing the Mongo instance, semaphore limit is large, as expected. Semaphore limit should be updated after construction if connectionsPerHost or threadsAllowedToBlockForConnectionMultiplier changes.