In the latest Zotero beta, we've added undo support.

You can now undo and redo many operations via Edit → Undo/Redo or the standard platform keyboard shortcuts. If, say, you overwrite the wrong field by mistake, or you drag a collection somewhere by mistake, you can just press Cmd/Ctrl-Z to undo the changes.

Not all operations are currently covered (e.g., deleting an annotation from the items list), but we'll be looking to extend support to more operations in future updates.

Unknown operations, such as many of the ones performed by plugins, are not undoable, though plugins can opt in specific operations. (We'll share more info on that on the dev list once this has been tested a bit more.) Some operations that change things in complex ways, such as a sync that modifies local data, may also clear undo history out of safety.

Let us know in a new thread if you run into any specific bugs.