There are any number of reasons for a corrupted file.

A file may be corrupted during transmission - while there is some error checking in place, it isn't too difficult for some corruption to occur here and there.

A file may be altered by someone - they may keep the same name, but modify the file in some way. For example, someone may fix an unplayable AVI by rebuilding the index. If you then download the file from this person and someone else, the files won't quite match (and hence be reported as corrupted).

For MP3s, AVIs, MPGs and other media files, a few bits or bytes of corruption will result in little more than a glitch in the playback. Not a major problem.

For ZIP, RAR, SIT, EXE, DLL and similar files, corruption is a real problem. You might as well junk the files as soon as corruption is reported.

There is no way to know ahead of time if a file is corrupted.

Mark