TEST(BSONObjBuilderTest, BadUseTest) {                                                                                                                                                           
    BSONObjBuilder root;                                                                                                                                                                      
                                                                                                                                                                                              
    BSONArrayBuilder array1(root.subobjStart("arr1"));                                                                                                                                        
                                                                                                                                                                                              
    array1.append(3);                                                                                                                                                                                                                                                                                                                                                 
    array1.doneFast();                                                                                                                                                                        
                                                                                                                                                                                              
    BSONArrayBuilder array2(root.subobjStart("arr2"));                                                                                                                                        
                                                                                                                                                                                              
    // Notice how we're using 'array1' here. It'd be nice if this tripped an invariant or something.                                                                                            
    array1.append(4);                                                                                                                                                                         
                                                                                                                                                                                              
    array2.doneFast();                                                                                                                                                                        
                                                                                                                                                                                              
    // This "just works" even though it doesn't produce the object you'd expect.                                                                                                                       
    auto obj = root.done();                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                              
    // ... later someone tries to use the object.                                                                                                                                             
                                                                                                                                                                                              
    std::cout << obj << std::endl;                                                                                                                                                            
    // Kind of a strange object, and clearly not what was intended:                                                                                                                                    
    // { arr1: { 0: 3 }, arr2: { 1: 4 } }                                                                                                                                                     
}