We show that short-term borrowing by intermediaries creates a rollover-coordination problem endogenously. We then introduce ambiguity and show that the coordination outcome is discontinuous in ambiguity attitude: starting from neutrality, any aversion implies a complete collapse for every value of fundamentals for which coordination matters, whereas any ambiguity loving causes successful coordination for all such values of fundamentals - a "lending euphoria." These help clarify credit booms even when backed by sub-prime assets as well as sudden credit freezes at the advent of bad news even for borrowers investing in non-sub-prime assets
