This morning I started a QuickStatements batch 18572, started to run it in background, discovered I made a mistake and clicked on "stop" after something like ~100 edits, then clicked on "revert the batch". Except that the batch didn't stop.
It visually didn't show me the stop button anymore, as if that had worked, but the numbers of edits continued to grow (and the edits were made). And the "revert the batch" part worked only for a very small part of the batch. I cleaned up everything this evening so there is no wrong data on Wikidata but I did make thousands of edits today which were wrong and which I thought I had stopped.