I have never thrown a WCG Error if my OC can pass 2 hours Prime95 Blend (use custom settings with max available memory).

WCG uses a nunber of different strategies to determine the "validity" of a WU. Some projects have internal checks so only 1 copy needs to be sent out and gets validated (or failed) immediatley after being returned. These projects also randomly send out a second copy 20% of the time to make sure you are still producing valid results compared to another machine. Some projectas start out by sending multiple copies (any where from 2 to 15) to check everyone's results against each other.

A last wrinkle that is not directly associated with validation but does confuse people is the HCMD2 project. As you process the WU if you are not finished at 6 hours of processing it checks to see if you are at least 60% complete. If you are not it stops then and the remaining pieces get folded into a new WU. If you have passed 60% it lets you continue until you are finished or you have processed for 12 hours at which point it will stop and the remaining pieces go into a new WU. In either case of a stoppage you will be credited for all of the work you have done. A side note to processing time is that is actual CPU time and not "wall clock" time. If you are running a newer version (6.20 or higher) the runtime listed in the Advanced View of BOINC Manager is "wall clock" time. If you want to see actual CPU time on a WU, select the WU and then click on the Properties button to the left.