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We were having this problem in a lab environment where user files are mounted via NFS. The client machines are running OS X 10.6.7 and the file server is running Debian Linux. After much grief and reading /var/log/syslog on the client machine I discovered it was a subtle file locking / race condition issue with the file system. Evidently, when editing files in a "bundle" (e.g. as in an Xcode project), the OS is using some form of "safe writes" which involves writing transient data atomically to ~/Library/Caches/TemporaryItems/ (which of course is on the NFS mounted system) -- there is a race here that requires proper file locking. We hope we have fixed the problem by changing the nfs_mount options as described in Mounting NFS volumes in OS X

Simply by rebooting the computer the issue was fixed.