Pulpo I had the same problem and replaced the pan a few months ago.
The new one already has a small stain way at the back of the water seal.
The only thing I can put it down to is the glazing is pretty poor these days, either incomplete or thin at best OR because they came out as a package from Caroma (pan and cistern for the price of the cistern only) I wonder if the pans are seconds??
Jack