A study of error detection in discourse is reported in which children 10 to 13 years old listened to set of expository texts read aloud, read the texts via a moving window simulated on a computer screen, or read them typed on paper. Occasional changes were introduced in the texts, resulting in illformedness at a semantic level, at a morpho-syntactic level, or (in reading) at an orthographic level. The subjects were 278 Swedish 4th and 6th grade pupils. Analysis of d' showed that all test groups performed above chance, that 6th grade children were better at the task, and that errors were easiest to detect while listening. By the same standard, detecting errors when reading was easier from hard copy. However, 4th graders detected both more othographic and morpho-syntactic errors when reading from the moving window, suggesting some advantage in presenting text to younger readers from left to right.