This is just my two cents but...
I got 7500 credit without a single error on 4 very different machines. Even on the one where I play civ4 and still run boinc at the same time :)

BUT - yesterday was running WMWare 5 with a xp pro image. Inside the image the CPU usage peaked at 100% (mysql server acting up), and bang, at the exact same moment - work unit errors poppping up in boinc.

Curious, I tried to reproduce it, and you bet, if the cpu usage shot up in the vm, I would start stop start stop boinc to try to free cpu usage and errors on the wus would appear.

So from that very basic and utterly unscientific test, I can say that WUs error appear if the vm cpu usage peaks, or if boinc is suspended/resumed/suspended/resumed in quick succession.

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