Cassandra's with Vivienne. This is absolutely unacceptable, and she will be heavily interrogating everyone involved to figure out exactly what happened and how to ensure that it never happens again. Once she finds out that it's not a rifter whose hand had acted up, but a mage attempting magic while in possession of a shard, she's going to swing heavily in favor of increased mage supervision (or at least Sina supervision). This has the potential to be A Big Deal and Cassandra can certainly make it into such.