According to the default-mode interference hypothesis, suboptimal performance in tasks requiring selective attention occurs when off-task processing (e.g., mind wandering) supported by default-mode regions interferes with on-task processing (e.g., attention) enabled by task-positive regions. In the present functional MRI study, we therefore investigated whether suboptimal performance in a selective attention task was linked to heightened interactions between a key default-mode region (the posterior cingulate cortex; PCC) and a key task-positive region (the left dorsolateral prefrontal cortex; DLPFC). We also investigated whether heightened interactions between the PCC and the left DLPFC were linked to enhanced future performance, consistent with prior data suggesting that such interactions index adaptive changes to the cognitive system. In line with both of these predictions, increases of current-trial functional connectivity between the PCC and the left DLPFC were linked to increases of response time in the current trial (i.e., suboptimal performance), but to decreases of response time in the next trial (i.e., enhanced performance). This double dissociation provides novel support for the default-mode interference hypothesis. Moreover, it suggests the possibility that, in at least some cases, default-mode interference indexes processes that optimize future performance.