While working on HOT Typhoon Haiyan mapping I noticed a whole area moved about 14m to the west while verify a job tile in task manager. Using JOSM I looked into the history of one node and found the changeset where the nodes were moved.
I noticed the user had only a few changesets. It could be that the user acciential moved the area. But I'm not sure, there may always be a reason. So I wrote a message to the user, tried to be polite and added the sentence «Under no circumstance this little critics should stop you from further mapping».
A few hours later the user responded with a elaborate answer, explaining he noticed an offset to the background and tried to fix it. He also noted that this wouldn't stop him from further mapping. This was my biggest concern that he was offended.
It seems very reasonable. In the changeset I saw he used the JOSM editor, so it's likely he accidentially added a aerial imagery offset. His other changesets all looked sane. I responded with some hints and offered to help.
We agreed that I fix it for him, so I've had my first case of a kind of revert. I checked the documentation to see what options are available. One seems to be the revert plugin for JOSM. Well I don't really wanted to revert the whole changeset, I just want to move it back.
Fortunately I found the «Changeset Manager» in JOSM («Window» Menu). So it was easy to enter the changeset number, load the whole changeset. Then I downloaded the affected area, big enough to surely include the whole changeset. Then I added the Bing aerial imagery layer as reference. Finally I opened the changeset manager again, clicked the button «Select the primitives in the content of this changeset in the current data layer», and voilà, all moved items are selected. Now I had just to move the items back, using the background image as guidance. Of course a dialog popped up, warning me I've had just moved more than 20 objects: Yes I'm sure! Finally I upload the new changeset and add a sane commit message.
That was easy.