Several times in the past few years, I’ve opened up my freezer to find that a 1″-2″ spike has shot up out of the ice tray. At first, I thought it was from dripping from above, then I theorized that it was where still-freezing water had ruptured through the ice membrane, bubbling up to form the stalagmite. I was all set to post an Ask MetaFilter when I encountered an ice spike this evening, only to find that there’s a whole webpage about them.
Live and learn.