Summarily, an entity will cast a second shadow one block under its own if over at least two blocks of non-opaque blocks. It must be at least two blocks of glass, stained glass, or regular ice but Not glass panes, tinted glass, honey, or slime blocks (blocks whose transparent insides / tops are not culled away in an internal view such like glass does). The two blocks can be mixed and matched from the list (though some combinations, such as glass on ice, look normal for there to be a shadow on). More layers do not seem to produce more shadows. This is not a duplicate of MC-21024 as those shadows are merely misplaced, but still properly numbered (though the edge cases between the two produce interesting results, such as the contiguous shadow in attached photo 18w46a.png). This behavior does not appear before 18w46a; I suspect it's been present ever since then. The set of photos with the cow and the dropped glass item are taken in a glass cage through spectator mode.