What the others have said is correct, it can be due to way too much radio frequency inteference within the Work Unit, and these are gifts for the stats
However, if a computer is spitting out lots of these work units (rather than just one every now and again) then there might be something wrong with the computer, such as not stable on an overclock (in which case you should knock the speed down), bad RAM (i have had this on a computer at my college, doing WUs in about 10 seconds) or some other deep down problem with the computer!
Confused