See the following images:

http://imgur.com/a/tE4Vx
http://i.imgur.com/pc331.jpg

What happens is that you "win" winterfest, but do not kill the gremlins attacking imposto right at the 0 second mark. The game keeps track of the gremlins even though they shouldn't be attacking anymore, causing the game to issue a lost sequence right after a win.

This is known from multiple instances to perma-glitch your UI until you restart spiral knights. It may not always do such a thing, but if timed right it will, as seen in the first imgur album.