Change rollback
Change rollback is a way of responding to vandalism and to 'mistakes' (which are not considered vandalism but which can be responded to with the same tools) to an older version of the data.
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Use editors to rollback a small number of nodes and ways
Potlatch
You can now use Potlatch to revert to an earlier version of a way, or to undelete deleted ways. See Potlatch/Primer#Undoing mistakes
JOSM
JOSM has 'undo' functionality which allows work to be reverted to an older state, but can only go as far back as the point at which the data was downloaded into the program.
If you already uploaded your changes, the Reverter plugin can be used to roll back a changeset.
Use scripts to rollback entire changesets
Introduced in API v0.6, a changeset is a group of edits made within a certain time by one user. It makes it easy to identify and deal with a problematic set of changes (such as a large-scale movement of nodes and ways or vandalism). Revert scripts use changesets in order to identify the changes to be reverted. However, changesets are not automatically revertable. Nor are all traces of the reverted changes removed from the database. Instead, the revert script will produce the same result as if someone examined the reverted changesets and manually changed all of the changed items back to the state in which they had been before.
Revert scripts are available to undo entire changesets, but should only be used if you know what you are doing. In fact most people should seek help on IRC or the mailing list, or ask the authors of the script to run them in a particular area. These scripts do not have safety nets. Be sure that you feel confident to fix anything you might break. Never do this unless you are absolutely sure that the edit in question is either malicious or accidental. When in doubt, discuss things on the mailing list before you act.
A quick note about clean and dirty reverts
A clean revert can be executed when none of the objects in the changeset have been changed in the mean time. In this case the revert will not have any side effects. A dirty revert happens if some of the data to be reverted is changed or used in the mean time.
Currently (as of 10 August 2009), the revert script refuses dirty reverts altogether by default. See the Original Changesets and Reverts Proposal 2008#Reverts and Revert scripts for more information. The JOSM reverter plugin can handle dirty reverts.
Change monitoring
When monitoring changes, it would be good to be able to revert malicious changes to the data (e.g. Vandalism).
