When rendering a frame, e.g. that is to be used for rendering subsequent frames to be rendered, two versions of the frame are rendered, wherein the first version of the frame is rendered in its entirety but only a portion of the second version of the frame is rendered. The rendered portion of the second version of the frame is compared to a corresponding rendered portion of the first version of the frame. When the comparison determines that the two portions of the two versions match, the frame is, e.g. used for rendering subsequent frames, but when the comparison determines the two portions of the two versions do not match, an error operation is performed.