Biggest lesson: all metrics _must_ be defined in code, not manager-speak.

For instance, if a marketing head wants to plot CAC (cost of acquiring customers) over time, saying CAC is number of customers divided by marketing spend is manager-speak. Spends are budgeted higher early in the month and adjusted with actuals. Customers ask for refunds and cancel accounts. Some campaigns have volume incentives which are known later... and so on. The solution is to write well commented SQL which laymen can audit and improve.