Suppose that a random variable $X$ of interest is observed perturbed by independent additive noise $Y$. This paper concerns the the least favorable perturbation $hat Y_ep$, which maximizes the prediction error $E(X-E(X|X+Y))^2$ in the class of $Y$ with $ var (Y)leq ep$. We find a characterization of the answer to this question, and show by example that it can be surprisingly complicated. However, in the special case where $X$ is infinitely divisible, the solution is complete and simple. We also explore the conjecture that noisier $Y$ makes prediction worse.
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