(→Acceptable reversions: advice to avoid edit warring)
In the case of a good faith edit, a reversion is appropriate when the reverter believes that the edit makes the article clearly worse and there is no element of the edit that is an improvement. This is often true of small edits. Edits that introduce [[wp:undue|undue weight]] should be reverted until consensus is built.
Whenever you believe that the author of an edit was simply misinformed, or did not think an edit through, go ahead and revert. If that editor (or anyone else) re-reverts, you will know it is more than that and
Another kind of acceptable reversion is an incidental one. A Wikipedia editor is not expected to investigate the history of an article to find out if an edit being considered is a reversion of some prior edit. The rule against reversions applies only to cases where the reverter is aware that the edit is a reversion of another edit.