I happened to me twice to day when try to d/l two nzb files, both time it skipped file .02 and .03 (rar format) => by the end it just say 100% complete without tried to do par repair (eventhought there are lot of paused par file left in queue) and of course it didn't extract them.
WinXP SP2 with FireFox 2.0.0.1 running at the time of d/l