I have a website that sells graphic collections. When someone makes a purchase we send out a zip file. 90% of the time they can download the file, but sometimes they get a zip with 0 filesize. The url they download the file from is "encrypted". When we get this problem I can directly link to the file and they can download fine. Does anyone know what might be causing this?

What method of 'encryption' does your script use? Does the URL include some code that has to match with something? Maybe its a bug in the decryption that only happens for 10% of codes?

I probably shouldnt have said decrypt. Basicly it sends them to an page with ie id=abcds in the url and in the database where it pulls the file from the id. Unfortunatly I didn't make this page so I'm not sure of all the lil ends and outs.

its neither encrypted or decrypted, its simply a mask. You are using a database that holds the physical path of the file, right? Check the id you are using in the URL against the record in the table you may be sending them to a record that doesn't exist.