Poltergeists represent a class of technohaunts given corporeal existence through the exploitation of angelnet security flaws. These tend to have been very high-level control programs, rendered obsolete by upgrades or replacements, but, in the classic technohaunt fashion, not completely obliterated from the system. They persist in the angelnet control systems, scattered blocks of code that occasionally interact to produce strange manifestations within the substance of the angelnet itself.
Poltergeists are differentiated from classical technohaunts in that their presence is very often unmistakable. They produce real physical distortions in the regions they plague -- weather abnormalities, "invisible" barriers made of misbehaving utility fog, "handlike" apparitions that reach out from thin-fog to manipulate people and things. Sometimes they simply pose a nuisance to the inhabitants of an angelnetted polity. Other times, they can be downright deadly as they misapprehend citizens as invasive agents and deal with them according to the perverted, fractured procedures of their damaged code.
Unlike technohaunts, poltergeists are easier to remove, since they tend to remain centered in control systems. This has produced a selective pressure on poltergeist programs, however -- those that survive the longest are those that are able to "spoof" legitimate programs, or those that can insert their code into legitimate programs for later execution, like a virus. Some poltergeists have developed extraordinarily sophisticated anti-detection schemes, leading some angelnet management experts to consider the possibility that these aberrations may be capable of intelligent thought.
If this is true, then the implications are chilling. Poltergeists have been known to persist in highly maintained angelnets despite the efforts of high-S expert systems to remove them. With the resources of an angelnetted world at their disposal, successful poltergeists are capable of evolving with extraordinary rapidity.
Some fear that poltergeists and other "anomalies" like them represent an emerging clade of high-S "accidents" that not even their transapient masters can fully control. For the time being, their populations are strictly controlled and their evolution closely monitored. Some have speculated, though, that modern angelnets may be the breeding ground for a new class of virtual being, as novel as the first AIs to emerge from the complex expert systems of the past.