An abrupt appearance of a new stimulus, or sudden onset, has several possible perceptual interpretations. The change may reflect an object new to the scene or instead be caused by disocclusion of a pre-existing object. Alternatively, the sudden onset may be interpreted as the morphing of a pre-existing figure (as in “line motion”). Previous work has focused on the morphing percept to the exclusion of other interpretations of sudden onsets. This paper supports the idea that morphing, and the other interpretations of sudden onsets, reflect occlusion cues indicating the most likely cause of the stimulus. Consider a line segment that appears abruptly. The data herein show that when the segment has already been represented as present in the scene(via amodal completion), its onset is likely to be perceived as a disocclusion event, with no appearance of morphing. Even when individual frames do not support amodal completion, dynamic (although motionless) cues can favor the disocclusion interpretation, again vetoing the perception of line motion. Some final demonstrations address sudden materialization, in which previously unseen objects suddenly appear. Again there is ambiguity in that sudden materialization and disocclusion can be caused by image changes that are locally identical. Remote cues to occlusion are shown to give these stimuli distinct appearances. The existence of these ambiguities, and the role of occlusion cues in resolving them, has implications for theories of motion perception and attentional capture.