Speakers often include extra information when producing referring expressions, which is inconsistent with the Maxim of Quantity (Grice, 1975). In this study, we investigated how comprehension is affected by unnecessary information. The literature is mixed: some studies have found that extra information facilitates comprehension and others reported impairments. We used an attentional-cueing paradigm to assess how quickly participants could orient attention to an object upon hearing a referring expression, such as the red square. If there are two squares differing in color, then the modifier is required. However, if there is only one (red) square, then the modifier is unnecessary. We also recorded event-related potentials (ERPs) in order to investigate online processing. Reaction times were significantly longer for referring expressions that contained extra information, and ERPs revealed a centroparietal negativity (N400) that emerged approximately 200-300 ms after modifier onset. We conclude that referring expressions with an unnecessary pre-nominal modifier impair comprehension performance.