I just got killed by an invisible mod. I have no mods whatsoever installed, just plain Minecraft 1.8 singleplayer in adventure.

Long story: I fed my animals and got surprised by 3 zombies and 1 skeleton jumping from a small cliff near my animal area. I killed the skeleton first, but was still attacked by something though I was still out of range of zombies, so I died. I respawned very near in my bed, went downstairs to forge a sword and was suddenly attacked again without any monster in sight. I ran up to my bedroom which is a completely enclosed room with two doors. I went to a corner of the room with perfect sight. I saw no monster and there was no chance of a skeleton hitting me with arrows, but something was still able to hit me and killed me again (after 3-4 hits, no armor). I quit Minecraft completely and loaded my world again. Then the attacks stopped.