Overconfidence leads one to seek leverage. Leverage in turn magnifies the consequences of errors. Leverage on LTCM's scale, which dwarfed that of its predecessors, turns errors into catastrophes.
Another, probably more important lesson concerns the degree to which one should rely on historical data. If something never has happened, or at least not recently, one cannot safely bet the house that it never will happen.
I probably shouldn't have speculated on what "popular belief" does or does not hold, but it has often been written, falsely, that LTCM was unhedged against Russia defaulting on its own bonds.
Inventing Money, by Nicholas Dunbar
Subject: The failure of Long Term Capital Management. See also gjm's recommendation of Lowenstein, but Dunbar supplies a briefer and more financially sophisticated treatment.
Lessons: The danger of leverage, which is one potentially very expensive form of overconfidence. The world contains more risks than anyone, no matter how clever, can think of to hedge against. LTCM, contrary to popular belief, did in fact hedge against Russia defaulting on its own bonds, through currency forward contracts. What they failed to hedge against was Russia then suspending payment on the forward contracts, and hiding its hard currency.
Linster I will not trouble to defend, but you are reading Gray uncharitably. By "more deadly" he surely means "duller" ("dull" is an adjective to which "deadly" is often yoked). The claim that immortality would be boring may be false, but it is not obviously ridiculous.