During a shoot I noticed a slight vignette on the top left and right corners of the LCD. I checked my lens was mounted correctly, checked for any obstructions, filter, grime, nothing. As panic slowly started to creep in I realised that somehow I had bumped the ND filter wheel and the edge of the filter was showing up in the image. I wish I had noticed earlier as now a good proportion of my images have this vignette effect as it was difficult to spot. Oops...