Stable diffusion is very good at continuing patterns, as the smaller image is enlarged the coherence of the original local areas are maintained this results in an accurate averaging of the area colour, any artefacts will be blended, the downside is that over iterations the algorithm might start to define unwanted areas as relevant and they will grow with time, another artefact might be the blending and un-sharpening of fine straight lines and edges, furthermore it might be different every time you run it dependent if you have access to the seeds.
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