I tried calculating a field with only some records selected. It calculated on the entire field completely. Had I not recently backed up the data this would have spoiled several weeks of fieldwork. Even with the backup I've spent the last several hours attempting to repair the data. Turns out that is a known limitation (https://support.esri.com/en/bugs/nimbus/QlVHLTAwMDExNzY3NA==), but didn't know and there is no warning... There is some limited undo support if you are on the edit tab of a web map, but if you're editing the attribute table directly and you make a mistake then you're screwed.
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