Zip file is always an issue, not only with zen, on an average of 10 zip files I download there will be at least 1 that has to be redownloaded.

Corrupted zip files are 99% of the time caused by what is called packet loss during the download process. This means that if your internet connection has even the slightest of hiccups during the download process, it will cause a millisecond disturbance to the data stream and your download will not be downloaded completely. So even if you get 99.999999% of the file, it still will not work.