I received a reply from SOM online support this morning:
We are trying to recreate this but haven't been able to so far. Is there anything that has consistently caused this problem? Any extra information you send would be helpful.
It seems random to me, but perhaps someone else can detect a pattern? I also let them know that, as Scumby reported, it isn't just affecting mystery cards.
Here's my browser and OS; please post yours if you're having the issue (at least until we can establish there isn't a patten with these).
Firefox 37
Windows 8.1