When mining, I have been creating well-lit, single-block paths of travel up near the ceiling of ravines. While doing so, I skeleton was presumably in a cave outside of the ravine, but ended up directly behind me. This has caused a major setback.

Another time was in an enclosed mining facility. It was well-lit with no entrances to caves. However, when I turned around, there was a witch. This area was known for having blocked-off caves not far from the walls.