ERP implementation: what the vendor demo never shows

The demo runs on clean data, a standard process and a consultant who knows the shortcuts. Your implementation runs on fifteen years of accumulated exceptions, three spreadsheets nobody admits to, and staff who have to keep the business running while they learn it. The decision that seems to matter most is whether you change the software to match your process or change your process to match the software. Both are expensive in different ways. Which did you choose, and what would you decide differently now?