The error you report occurs when the cluster tips cannot erased because Windows has locked the file that resides immediately before that tip area. They're probably system files that Windows loads at boot up. It's not a major problem because those files would have probably been locked when Windows was first installed so there shouldn't be any remnants of sensitive files in the tip area (no other software has had permission to write there).
Some background: hard discs have a minimum 'file' size called a cluster and the 'cluster tip' is the space on hard disc between the end of a file (that has not been deleted) and the end of the cluster.
Say your cluster size is 8KB. If you save a 8KB file it will take exactly one cluster of disc space but a 1KB file will also take up 8KB - the 1KB file and an empty 7KB cluster tip. A 12KB file will take 1.5 clusters.
The reason to wipe the cluster tips is because a new file saved on top of a deleted, but not erased, sensitive file will have overwritten some of the sensitive file, but other parts of it are still able to be recovered from the tip area, which has not been overwritten.
