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In fact, we don't know what actually causes them. It's been rather hard to pinpoint as to what actually creates them, but it's definitely server-side. "Ghost" or phantom workunits have been happening since long before the recent server issues, so it can't be blamed on that.
My educated guess is that the server creates the workunit, puts it in a queue to be downloaded, then a machine requests more work. For some reason, the server assigns the task to a machine, thinks it sends it but the process fails.
This seems to be something unique to the BOINC server software since a feature was added to specifically reduce ghosts on user's machines called "Resend". Resend checks the user's machine for all their workunits and compares it to the server's list of what it's supposed to have and if any are missing, it simply resends those workunits.
Problem is, the resend feature is very database intensive, especially for such a popular project like SETI@Home. Since SETI's database can hardly handle the normal traffic at this point, the features has been turned off. Hopefully, when they get some faster/newer hardware, they might be able to turn it on again. We'll see. | 
