Texture bleeding
Colour from neighbouring pixels leaks across the edge of a tile or sprite when the texture is filtered or mipmapped.
When the GPU samples a texture with bilinear filtering it blends in nearby texels, and mipmapping shrinks the whole atlas until separate cells merge. The result shows up as thin seams or wrongly coloured lines between tiles in a tilemap, especially in motion. The problem is nearly universal once you pack many images into a texture atlas.
The cures are all about margin: add a border of extruded edge pixels (a "gutter") around each cell, inset the UV coordinates by half a texel, or disable mipmaps for the atlas. Pixel art is often rendered with nearest-neighbour filtering precisely to avoid the leak.