This paper studies hidden arbitrage opportunities in markets where large traders affect the price process, and where the market is complete (in the classical sense). The arbitrage opportunities are “hidden” because they occur on a small set of times (typically of Lebesgue measure zero). These arbitrage opportunities occur naturally in markets where a large trader supports the price of some asset or commodity, for example corporate stock repurchase plans, government interest rate or foreign currency intervention, and price support by investment banks in IPOs. We also illustrate immediate arbitrage opportunities generated by usual market activity at specific points in time, for example the issuance date of an IPO or the inclusion date of a new stock in the S&P 500 index.