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Word. Especially when your internet connection isnt fast and you are doing other stuff on the connection. There is always a chance that some libraries didnt download or didnt download fully. For macs, sometimes because the POM package is so bug, it simply fails, needing a full download of the whole thing.

Also, some things, like XP SP3, which is HUGE, will fail to download, say, when installing IE6 (I think. I know that there are a few that require this package for related packages).