While playing, sometimes the client will not load skins. It may be the player's skin, or another player's skin, or a player head. The skin can show up on a player, but not on a head and vise versa, and may or may not be accompanied by an error output in the dev console.
This is seemingly random, and the effects are different each time the client is restarted. Sometimes, your skin loads, but no one else's, sometimes the reverse, sometimes random users don't have their skin. This is CLIENT SIDE ONLY, implying that the problem is not with the skin servers.
Tested in single- and multiplayer