<p>Source amnesia refers to the failure to remember the source format of information despite remembering the content itself. While well-documented in long-term memory, recent studies have revealed that source amnesia can also occur in short-term or working memory. Across four experiments, the present study aimed to investigate why short-term source amnesia arises, focusing on whether it results from source misattribution between items or item-specific interference caused by repeated exposure to the same content in different formats. We found that source misattribution persisted even for a single item presented per trial, suggesting that item–source misbinding between simultaneously presented items is not necessary for source-amnesia effect. Source misattribution was significantly reduced when the test item was novel or had consistently appeared in a single format across trials, but reliably emerged when the same item had been presented in different formats. These findings suggest that short-term source amnesia reflects item-specific source misattribution, driven by the coexistence of conflicting source traces for the same content. We propose that the task-irrelevant source information for target stimuli is stored in an intermediate representational state—activated long-term memory—which maintains weak bindings to its content but lacks robust contextual indexing.</p>