If you take a typical 4L jug of milk and lay it on its side in your refrigerator without first making sure the lid is tight (right from the grocery store), it can and will slowly leak. I see this all the time. We buy a new 4L jug before the opened one is empty (kids go through a lot) and there's no room in the fridge to stand it upright.. The place we can fit an upright 4L jug is already occupied by the opened 4L jug, the applejuice box, etc.

So... it's not normal procedure to buy the new jug and just before we lay it on its side on the fridge shelf we check the lid is tight.... and no leaks.. forget and there's a small puddle in an hour or so.