when using RequestStart(), ReleaseConnection tries to return in-use connection back to the pool resulting in NullReferenceException as Connection has been closed as the pool reaches maximum size. To reproduce, do something like

var db = server.GetDatabase("test");
var collection = db.GetCollection("test");
collection.RemoveAll();
using (var x = db.RequestStart())
{
for( int i=0; i<1000; i++ )
{
collection.Insert(new BsonDocument {

}, SafeMode.True);
}
}