Hello all. I was doing a few arena matches, and played a shard of serendipity when this cropped up. One of the cards came up labeled as 'undefined', and would spam an error at me in the chat whenever I hovered the pointer over it. I've played lots of Serendipity shards before, and I haven't seen this occur before.
Oh, and attempting to play the undefined creature caused my and the opponents health bars to disappear for a turn, and caused a blank space in my creature field. The golden goose I played right after it didn't show up until the next turn either.