The question of interest in this paper is the estimation of the trend of a financial asset, and the impact of its misspecification on investment strategies. The setting we consider is that of a stochastic asset price model where the trend follows an unobservable Ornstein-Uhlenbeck process. Motivated by the use of Kalman filtering as a forecasting tool, we address the problem of parameters estimation, and measure the effect of parameters mis-specification. Numerical examples illustrate the difficulty of trend forecasting in financial time series.