Broken hardware is my bet. Several years ago, in a bit of a convoluted setup involving a big room, several bookcases and a troublesome place where the wire came out of the wall, I had a router that balanced on a bunch of heavy law books. And when cleaning, someone apparently made it fall off those books before putting it back as if nothing had happened.
The results were extra-ordinary: rather than have the network connection die or anything of the sort, stuff just started getting garbled. Websites had garbled images with random lines and color shifts and the sorts; HTML documents had random characters happen and so forth. So somehow this entire affair happened somewhere inbetween the layers of error-correction; I suspect the router had its memory or cpu get messed up, but had its networking interfaces still work peachy.
Perhaps your situation is similar in nature here.