Windows 8.1 and OneDrive changes
Windows 8.1 introduced significant changes in how OneDrive (previously known as SkyDrive) works, and one of these is the possible replacement of actual files in any dedicated OneDrive folder with placeholders, which MS calls Smart Files.
These placeholders deliberately mimic real files (they have the same properties) so that if you have a profile that scans such a OneDrive folder, it may now think files are there (and maybe changed since 'last time' or 'now out of date' - depending what it is comparing them with) and try to copy or update them, leading to errors (because the real files are not really there). You may need to open Windows File Explorer, right-click on "OneDrive" in the left-hand pane, and click "Make available offline" to remedy this.
For Windows 10 users, refer to Windows 10 and OneDrive Files On-Demand articles.