Hydronic heating system[1] (hot water running under the floors) that was incompetently installed. No expansion tanks, wrong pipes used (no oxygen barrier so oxygen seeped in and rusted the cast iron pump), pressure wasn't balanced, just tons of issues.

My current house's HVAC system was also done piss poorly in places and there can be a 15 degree difference between rooms, so that's going to be 10k+ to fix. Fun times.

[1]Despite the obscene maintenance costs, the luxury is worth it. Warm floors are really nice!