We consider the problem of encoding an i.i.d. source into a set of symbols or messages that may be altered by an adversary while en route to the decoder. We focus in particular on the regime in which the number of messages is fixed while the blocklength of the source and the size of each message tend to infinity. For this fixed-blocklength, “large alphabet” channel, we show that combining an optimal rate-distortion code with an optimal error-correction code yields an optimal overall code for Gaussian sources with quadratic distortion and binary uniform sources with Hamming distortion but that it can be suboptimal by an arbitrarily large factor in general. We also consider the scenario in which the distortion constraint that the decoder must satisfy depends on the number of errors that occur. We show that the problem can be reduced operationally to one with erasures instead of errors in two special cases: one involving lossless reproduction of functions of the source and one in which the encoder and decoder share common randomness.