As I mentioned a few days ago in twitter, the outflow pipe of our sump pump froze.
I had gone down stairs to get something from the utility room and found that the sump pump was running … and didn’t stop. The pump sounded OK, but no water was draining from the pit. I turned the power to the pump off and triggered the backup pump … which ran but didn’t remove any water. This meant that the pumps themselves weren’t the problem. I went outside and discovered that the outflow pipe was frozen solid. Clearly not a good situation.