Graphical User Interface (GUI) testing literature emphasizes testing a system's functionality through its GUI, rather than testing visual aspects of the GUI itself. In this paper we introduce the notion of visual testing as a subset of GUI testing. To explore visual testing, we have conducted a study of defects in four open source systems. We found that visual defects represent between 16% and 33% of reported defects in those systems. Two categories of visual defects are identified with six subcategories within each of them. Other findings are also reported that are aimed at motivating the importance and the need for systematically conducting visual testing among researchers and practitioners.