For what it's worth, here's my abbreviated story:
At our semi-annual inspection on Nov 5 our furnace was tagged (shut off) and we were told we needed a new heat exchanger. It's under warranty, so the new heat exchanger would be free, and we would only have to pay labor to install it. We opted to do the replacement rather than put in a new furnace. The heat exchanger replacement was scheduled for November 13. We waited a week without heat for 8 days. Then, at 4:45 pm on Nov 12, we received a phone message saying they had just then reviewed their employee's Nov 5 work report and realized additional parts were needed, i.e. a diverter box is also needed, which they did not order and don't have on hand , so they cannot do the repair on the scheduled date, and furthermore, the cost of repair will be twice as much. From my perspective, we endured a very cold house for a whole week for nothing. Now we are back to square one. This needless suffering occurred because the hvac company didn't review their own paperwork when they planned the repair job and ordered parts.