I was just checking out the new UI when I go and talk to Maskwell,to see what items he has to offer
I go ahead and preview them,I accidentally preview the prize box.Then I try and exit my knight(It's on preview) but it wouldn't let me...I could chat but I couldn't close preview nor could I log off until finally the cursor turns pointy again,so I log off but then my game freezes.I press the "start" button to exit Spiral knights on fullscreen and manually close spiral knights but it didn't work.The last thing I did was reset my comp....

TL;DR
Previewing prizeboxes from a vendor prevents you from closing preview :/