I have a rough recollection of this happening earlier today but I haven't made sure that it's actually true. Filing it anyway in case it is. If you have an index, and iterate over records in its object store with a cursor retrieved from index.openKeyCursor(), calling cursor.update() triggers an exception that complains about the transaction being readonly. Instead, it should trigger an InvalidStateError that mentions that cursor.update() only works on cursors obtained with index.openCursor().
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