I'm a developer and as part of the build process, a Microsoft dll is copied to a certain folder. That file copy is now failing as the target can't be overwritten.

I decided to delete it by hand (using an admin account but a non-elevated explorer) so browsed to the folder and attempted a delete. This failed (Require permission from the Administrator). The same applies when using an elevated explorer.

So I tried Properties->Security->Advanced->Ownership

The current owner is showing as Unable to display current owner. I can't take ownership (a simple Access Denied message with no elaboration). Elevated Command Prompt/PowerShell don't help either (both give an Access Denied in their own way).

Process explorer shows no open handles on the file.

Eventually, I booted to linux and deleted the file but what I'd like to know is what caused it?

Security Essentials had no issues with the file. It's digitally signed by MS and the signatures match.