My world was about 15 minutes old. I went into a cave, where I encountered a skeleton. While trying to kill it, I heard a creeper start charging up next to. The creeper's charge-up process seemed to cause a massive lag spike that prevented me from doing anything (this lag spike happens to me all the time with creepers, so I'm not sure how relevant it is), but once the respawn screen came up, I could still see the skeleton. When I came back to retrieve my items, I could hear the skeleton, but not see it. It started shooting at me, but I couldn't find it. I ran and hid around corner. At this point, I could see every possible hiding place in front of me, but I was still getting shot from nowhere. I died again, and when the respawn screen came up, the skeleton just sort of appeared from out of nowhere doing a jittery glitch-bounce up and down.