Exactly what the title says. If you build a burough in a city, have the buroughs pillaged, and then re-build the buroughs, they apply their debuff to the city's improvements again. Because of this I know have farms in my city that only provide 1 production, and a rice field that only provides 1 faith.

I did also lose and re-take the city during this whole process, so that might have had something to do with the bug. Love the mod!