Files disappear, only to reappear mysteriously a year later, in another directory.
You know it's guilty of trashing your system, but you just can't prove it. Suddenly appears from nowhere, promises to upgrade all your applications, gains control of your computer, then totally corrupts your entire system for four years. Your system works fine, but it complains loudly about foreign software. Activates every component in your system, just before the whole thing quits. Makes its presence known, but doesn't do anything.