A corpse isn't buried promptly after its funeral, and a pregnant cat jumps over its unattended coffin.
The corpse returns to life, still dressed in its formal funereal attire. Stiff from rigor mortis, its arms are permanently stretched in front of it, and it can only move by hopping - perhaps a slightly comical sight, but it stalks the night to drain the vital force of the living. The body has become a jiangshi.
There are legends around the world of the dead returning to life, with details that emphasize different aspects of the innate and visceral human fear of death. The jiangshi of Qing-dynasty China is one such example, its form presenting the particular features which distinguish a fresh corpse from a body that is still supposed to be capable of motion. The medieval revenant is, of course, another.
Greece has a tradition of the vrykolakas, a person returned from death by having lived an ungodly life or being buried in unconsecrated ground. For a time this belief was sufficiently widespread that a custom arose of exhuming every body after three years in the grave, examining it for decomposition, and blessing those which were suitably rotten with consecrated wine for reburial while destroying the fresher-looking bodies as one would a confirmed vrykolakas. The nearby Slavic peoples, of course, have the vampire, which greatly resembles the revenant of earlier western Europe - however, different historical sources of vampire mythology seem to indicate a much more complex and developed form of the myth, often varying by region. One version, for example, specifies that they take forty days from death to become a vampire; in some sources, children born with cauls or tails are doomed to later rise as vampires after death, while in other sources those born with tails are likely to be werewolves while vampirism has other causes. Slavic vampires are also specifically associated with blood-drinking, with repulsion by crosses or garlic, with a compulsion to count small objects thrown into their path, and other familiar elements used by the more traditional of the modern takes on the form.
In Scandinavia, the medieval legend of the aptrgangr (basically a revenant) has come to be specifically associated with the returning bodies of dead sailors, giving rise to the modern folkloric draug - the returning body of a drowned sailor plaguing the living, often with his head either missing or replaced by one made of seaweed.
Among the Roma, an old legend has that those who die suddenly and unnaturally or who are not given proper funeral rites can become a mullo (literally "dead person") and rise to harass those they disliked in life. The mullo is also likely to attempt to strangle the person they blame for either causing their death or failing to perform the proper rituals of burial.
The pattern throughout old-world undead legends of a proper burial being a necessary (but not always sufficient) step in preventing the dead from returning indicates the close tie to the fear not of being dead but of dying, and of corpses as a source of defilement. A proper burial symbolically marks the transition from life and the end of life to eternal rest, and when it is skipped, there's a level on which the living may feel a lack of closure and the death may be seen as insufficiently final. The myths of the undead, therefore, represent a preoccupation on the part of the grieving with the need to move on, and the inability to do so in some instances.