A week or so ago suddenly I stopped being able to undo in the sample editor. It says it undo the action in the status bar but the waveform is not reverted to it’s former state. I’m on aMacBook Pro running Yosemite. I guess it’s lucky that I’m so rad mad at beat making that I don’t need undo; but it still. It’s broke. 
Sounds like in the bottom left corner of the sample editors waveform view, the undo box is ticked of?
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Undo/Redo Support
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- Enable/disable undo in the Waveform editor. When working with very large samples, temporarily disabling undo may be useful to save time. All undo/redo processing is saved to disk, so running out of memory should not be a problem.
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Also it’s possible to break it with scripting so that you can’t undo past a certain point and it would say ‘nothing to undo’ even when that field is ticked.
Sounds like in the bottom left corner of the sample editors waveform view, the undo box is ticked of?
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Undo/Redo Support
- - Enable/disable undo in the Waveform editor. When working with very large samples, temporarily disabling undo may be useful to save time. All undo/redo processing is saved to disk, so running out of memory should not be a problem.
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My hero! <3