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I have seen this as well. It occurs when Poisoned is
attempting to copy the completed download from the "temp"
folder -- wherever that is -- to the PoisonDownloads
folder. Sometimes this fails, and the file simply
disappears.

As you noted, it seems to happen primarily when the system
is "busy". However I have noted that the drive definitely
does something, so there's the possibility the file is
ending up somewhere else on the hard drive, as opposed to
failing outright.