We show that the frequent claim that the implied tree prices exotics consistently with an arbitrage-free market is untrue if the local volatilities are stochastic. This is a consequence of the market incompleteness under stochastic volatility. We also show that the problem cannot be mitigated by conveniently defining some 'weakly stochastic' local volatility, as this would violate the no-arbitrage condition. In the process of constructing the proof, we analyse — in the most general context — the impact of stochastic variables on the P&L of a hedged portfolio. We find that any stochastic tradeable either has quadratic variation — and therefore a Γ-like P&L on instruments with non-linear exposure to that asset — or it introduces arbitrage opportunities.