Client can't open FCP generated MP4 in Windows
 

This may be the wrong forum to ask this question but a client can't open an MP4 file that was generated in FCP. Could this be a problem with her version of Quick time? The eror message she gets is "Error -2048. Couldn't open the file._FOB-final.mp4 because it is not a file that Quicktime understands." Would an upgrade of the clients Quicktime solve the issue?

That "._" prefix is ominous.

You sure she's not trying to open a shortcut that OSX leaves lying around on a Windows disk? Is there a similarly named file (but without the "._" prefix) in the directory? Does that open?

When you copy a Mac file over to a Windows file system (FAT or NTFS) then two parts are seen on Windows a filename.extension & a ._filename.extension The .- contains the resource fork, along with any metadata like Spotlight comments & any icon preview etc. are stored in the ._ files. These ._ files are redundant on Windows & normally are invisible as they are hidden files. If the Windows user has enabled 'show hidden files then as in this case it can cause confusion.

It's best to remove them before sending out the files if you know that the target computer is a Windows PC & that the user will get confused. There are utilities that will do this for you e.g. BlueHarvest: Manage DS_Store, resource forks, Trashes and more. but I have never used them. In OS X Leopard from terminal there is the dot_clean command. man dot_clean for details of use.

Nigel's spot on.

I always tell people to look at file size - it never lies.
If your client is trying to open something that is 3k and your file is 30 mb, there is a problem. The only exception to the file size rule is a reference file but these usually contain audio, so it still would be larger than a shortcut or a DS_Store file.

When I deal with Windows clients, I always play it on a PC before I send it.
This is good practice if you use FCP. If you cant test, send a WMV (flip4mac) in addition to the MP4.
-C

I usually encode both for the client reviews in the case that some are viewing on a PC or view on MAC.