On every card you can click on the Changes tab at the top-right corner of the card to see the history of edits to the card, and revert to an old version if you like.
Examples
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How To
When you click on the Changes tab, you'll see something like this:
Newer and Older take you forward and backward through different versions of the card. Normally, deleted parts will be red and struck through, and newly added parts will be green. If you'd rather just see that version as it is, click on Hide changes. Clicking on Save as current will replace the current content of the card with the version that you're looking at.
Tips
If someone was editing a card but didn't save it, a copy of what they were working on will be accessible from the Changes tab. See auto save.
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