(Don’t know if this has been fixed in recent Outlook versions, we use 2002 here…)
An employee called and explained the following error to me: He was trying to open different attachments from his sent mails and got randomly a “file not found” error. After some research I found out that the filenames of most attachments were “Angebot.pdf” (the German word for “offer”). So I opened up ProcMon on his system, set the filesystem filter, did a search for “Angebot” and finally saw this:
595 15:10:31 OUTLOOK.EXE:2140 CREATE C:\Dokumente und Einstellungen\username\Lokale Einstellungen\Temporary Internet Files\OLK121\Angebot.pdf NAME COLLISION Options: Create Access: 00120196
596 15:10:31 OUTLOOK.EXE:2140 CREATE C:\Dokumente und Einstellungen\username\Lokale Einstellungen\Temporary Internet Files\OLK121\Angebot1.pdf NAME COLLISION Options: Create Access: 00120196
597 15:10:31 OUTLOOK.EXE:2140 CREATE C:\Dokumente und Einstellungen\username\Lokale Einstellungen\Temporary Internet Files\OLK121\Angebot2.pdf NAME COLLISION Options: Create Access: 00120196
...
693 15:10:31 OUTLOOK.EXE:2140 CREATE C:\Dokumente und Einstellungen\username\Lokale Einstellungen\Temporary Internet Files\OLK121\Angebot99.pdf NAME COLLISION Options: Create Access: 00120196
So Outlook tries to save a temporary copy of the file in temporary internet files and when a file with the same name already exists, Outlook is adding a number to the filename. In this case it looks like the user had not cleaned up his temporary internet files for quite some time. And above from that Outlook 2002 can only count up to 99 and then fails with the “file not found” error 
UPDATE:
Cleaning up the temporary files isn’t the solution. Outlook’s own folder for storing temporary files (attachments and such) is stored in the variable OutlookSecureTempFolder in the windows registry (HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security in my case). So you have to clean up this folder which can easily done by a logon/logoff script and this little tool.
