It is likely that some reflections from the moon are affecting your data. These reflections are unpredictable and have been found to be more frequent when the target is closer than ~40deg to the bright moon. You might also want to check http://www.eso.org/sci/observing/PublicSurveys/useful_links/OmegaCAM_images.html where we provide a collection of example OmegaCAM images, illustrating some special data features.

A comprehensive description of features visible in OmegaCAM detectors can also be found in the quality control web pages.