your lucky home depot messed up only one cut. I would never recommend having them cut the wood for your projects. it's just not exact enough. you can use hardwood shims to fix it. place them thick end in from the inside and trim them on both sides. Ideally I would do this the whole length of the gap. You are also lucky it was on a side, if it was the top or bottom you'd have to get a new piece of wood.
I was thinking of just cutting a length of Ply 1/4 think and use that a full shim.