The single biggest mistake is building your budget around wishful thinking instead of the data-driven reality of your business. So many brands start with an ambitious revenue goal and work backward, creating a plan that's disconnected from what's actually possible. This sets you up to either burn cash chasing unprofitable customers or feel like you're failing when you miss an arbitrary target.
The most common version of this is what Taylor Holiday and Richard Gaffin of Ecommerce Playbook call "ambition-based budgeting." You fall into this trap because it feels intuitive to start with your goal. But it's a costly mistake. You end up judging your marketing based on whether it hits a top-line revenue number, not on whether it's actually generating profitable growth. The fix is to flip the model. Start with your historical data on ad spend and new customer acquisition efficiency (aMER). By building a simple regression model, which the hosts break down in detail, you can see the real relationship between your inputs (spend) and your outputs (new customers and revenue). This creates a forecast anchored in fact, not hope.
Another huge error is treating every month the same, completely ignoring seasonality. Your customer's buying behavior and your ad costs aren't static throughout the year, so your efficiency curve isn't either. People make this mistake for simplicity's sake, but it leads to major misallocations. You'll likely overspend during slower periods and underspend when demand is highest, leaving money on the table. As Luke Austin often points out, you need to expect different results in February versus November. The simple fix is to build models that account for seasonality. On Ecommerce Playbook, the hosts explain that even modeling by quarter or by month, using historical data from those specific timeframes, can give you a much more accurate map to profitable growth.
Many brands also get stuck waiting for "perfect data" before they start building any model at all. This search for certainty is understandable, especially with fuzzy attribution, but it's a form of paralysis. The cost is that you continue operating on guesswork and gut feelings, which is far riskier than using the directional data you already have. In the "Stop Guessing Your Ecommerce Budget" episode, the hosts make the point that you need to move forward with the information available. The fix is to start now. Use your platform data, your sales history, and your marketing expenses to build a basic Spend & aMER model. It won't be perfect, but it will immediately give you a more intelligent framework for making decisions than you had yesterday.
Finally, a classic mistake is creating a spend forecast that's completely divorced from your marketing calendar. Your model might say you can spend a certain amount at a certain efficiency, but it's meaningless if it doesn't account for your planned promotions, product launches, or sales events. These activities are what actually drive revenue. The cost of this error is a plan that finance believes but marketing can't execute. On the show, Richard and Luke talk about using an "event effect" model to fix this. It's a way of quantifying the lift from your marketing activities and layering it on top of your baseline spend forecast. This transforms a static spreadsheet into a real, dynamic revenue model.
Building a good spend model gives you a clear, actionable plan that connects your marketing spend directly to profitable growth.