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Daniel Barrett wrote:

On an article's history page, there are "undo" links next to every single revision. However, clickubg some of the undo links displays:

"The edit could not be undone due to conflicting intermediate edits."

Two questions:

1. What is the precise rule for when an undo can/cannot be done? When

there have been two edits to the same line? Two edits by two different authors to the same line? Etc...?

It attempts an automatic reverse diff application via GNU diff3. If this fails, then it tells you it cannot be done.

Failure would be caused by some intermediate change causing a diff application conflict.

2. Why does MediaWiki display an undo link next to a revision when the

operation is not possible? Is it because it's too expensive to check the "undo status" of every revision when rendering the history page?

Yes.

- -- brion