There are lots of hosts at Citizen Science Grid with erroring or aborted workunits. Then when they error/abort, legitimate users can't get their results validated properly and therefore lose credit. Also, these hosts drain the WU cache , keeping others from receiving fresh WUs.
Check out the CSG message boards for some example bad hosts:
Most are running BOINC version 7.0.80 (!) which means... Charity Engine.
I wonder how many aborted WUs are frustrated users killing tasks when CPU is pegged at 100%?
So much wasted crunching power! So annoying!