From my experience, this mostly comes from broken downloads, means the download was terminated (for whatever reason) before the file is fully stored on your hdd.
If you have a multi-segmented rar-file (part1.rar, part2.rar,part3.rar) all the segments (except the last one) sould have the same file-size.
If your download consists of one or two parts, the first one should be bigger than the second (last) part, maybe try to compare the file-size of your download with the size that is given in the preview.
Just try to download the files again.
As far as i know it makes no difference if you use a mac and the file is packed on a windows pc.
Of course it can be that the files have been corrupted before uploading them, but this is a rare case imo. And it can happen that the files went corrupt during the upload session. If you downloaded the files again and the problem persists, pm the uploader so he can check his files.
But i would say that 90% of these errors result from corrupted downloads.
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