I had it happen a couple years ago. Had just lit the fire and stuck my hand in the flue pan while it was still cold - and came out with a handful of snot! I ran the garden hose into the pan while I let the fire go out, drained the pan and pressure washed it. A couple hours later the pan was clean and I was boiling with fresh concentrate which made good syrup. The contents of the syrup pan were fine - it was the flue pan that turned.

This past Sunday I drained 400 gallons out of one of our sap tanks in the woods but left 20 gallons in the tank that was pretty nasty looking. By the time I'd sloshed it around to clean up the slime it the tank it had gone totally snotty and would barely drain! 15 gallons of clean water later the tank looked like new and the sap coming in was great.