It happens when a resource is localised on the disk, after localising that disk has gone bad. NM keeps paths for localised resources in memory. At the time of resource request isResourcePresent(rsrc) will be called which calls file.exists() on the localised path.
In some cases when disk has gone bad, inodes are stilled cached and file.exists() returns true. But at the time of reading, file will not open.
Note: file.exists() actually calls stat64 natively which returns true because it was able to find inode information from the OS.
A proposal is to call file.list() on the parent path of the resource, which will call open() natively. If the disk is good it should return an array of paths with length at-least 1.