A brief study has been conducted to investigate several visual anomalies reported by test pilots using a Display Night Vision Goggle (DNVG) that superimposed symbols onto the intensified image seen by the right eye. A survey of relevant research suggests that one oddity, an apparent focus mis-match between the scene image and the injected symbols, is an irremovable facet of the perception of bright, contrasting, overlaid symbols. A second oddity, an uncomfortable and distracting blurring of the under-stimulated left eye during periods of flight in cloud, was eventually experienced by several people in a laboratory simulation, the effect being more noticeable if the under-stimulated eye was the dominant eye. A subsequent apparent enlargement of the HUD symbols and a post-flight focussing delay by the left eye seemed to be after-effects of whatever caused the ocular discomfort. As about 30% of the population are left eye dominant, the disturbing discomfort and aftermath could affect this proportion of pilots using a right-eye DNVG. Although further work is needed to understand the phenomena, it would be wise to warn aircrew and enable the symbol injection unit to be fitted to either channel of the DNVG.