When we make errors, we tend to experience a negative emotional state. In addition, if our errors are witnessed by other people, we might expect those observers to respond negatively. However, little is known about how implicit social feedback like facial expressions influences error processing. We explored this using the cognitive control phenomenon of post-error slowing: the tendency to slow the response immediately following an error. Adult participants performed a difficult perceptual task: estimating which of two lines (horizontal or vertical) was longer. The background showed an irrelevant distractor face with a happy, sad, or neutral expression. Participants slowed after errors only when the subsequent distractor face was happy, but not when the subsequent distractor was sad or neutral nor when a happy face followed a correct response. This suggests that information about others’ affect, even non-interactive, task-irrelevant information, has performance- and valence-dependent effects on adaptive cognitive control.