Apparently, the water heater was jealous of all the attention that the shower has been getting. Last night, I went downstairs to find this:
Yep, looks like somebody's trying to get my attention. Walker has been tasked with finding a plumber to replace this beast when he gets home from work today. Just to make sure the other mechanicals stay happy, I scheduled a maintenance check for the furnace, too. I think I'll just stay in bed tomorrow.