What is Spenser's biggest flaw in the novel, Hugger Mugger?

Spenser is physically up to most tasks, being an ex-boxer who tries to stay in shape and knows a good deal about hand-to-hand combat and the use of various weapons. His biggest flaw, however, is the hubris that such toughness can engender, and even though his high opinion of his physical prowess is often a source of humor, Spenser nonetheless gets himself into some very tight spots by overestimating himself and being overconfident in his physical abilities.