We're using this technique to stuff the file descriptor with the accompanying object for the epoll registration, so one can check if the (reusable) object was reused for something else while registered with other file descriptor in the epoll: https://github.com/sociomantic-tsunami/ocean/blob/ee4f78d527...

LuaJIT is where I originally ran into this. It uses both NaN tagging with C doubles and tagged pointers with 64 bit pointers.