Sentence:  [A]n ambitious supersonic carrier fighter called the F3H Demon . . . proved too heavy for its Navy-specified Westinghouse engine (in itself a <WSD>problem child</WSD>), and turned into a $265 million fiasco. 
Meaning of the word marked with <WSD> tags:  Someone or something persistently difficult or vexing; a frequent source of trouble or annoyance.