An account that counts the wrong events will optimize toward them with total commitment. This is the failure I find most often and the one that costs the most, because it is invisible from the dashboard. The graphs go up. The reports look healthy. The phone does not ring any more than it did before, and nobody can explain why.
Here is what I regularly find sitting in the primary conversion column, all of them counting toward the number that Smart Bidding chases.
Any one of these will pull bidding toward the cheapest audience that produces that event. Cheap events attract cheap traffic. The account learns to buy people who load pages and never buy anything.
A conversion should be an event that a human being at your company would recognize as a lead. A submitted form with contact details you can act on. A phone call that lasted long enough to be a conversation, not a wrong number. A booked appointment. A completed purchase. If your sales team would not put it in the pipeline, it should not be a primary conversion.
The primary and secondary distinction is the tool for this. Primary actions drive bidding. Everything else goes to secondary, where it is still visible in reporting but does not steer the machine. Most accounts should have one or two primary actions. If you have seven, you have no target at all.
Some businesses cannot cleanly track the thing that matters. A restaurant, a walk-in clinic, a shop. If the sale happens offline and you have no booking system, you are measuring a proxy no matter what you do. In those cases I prefer a small number of honest proxies, calls over sixty seconds and direction requests, and I set expectations that the account will be steered partly by what the owner observes in the business rather than by the platform alone.
Low-volume accounts have a different problem. If you get a handful of real inquiries a month, that is not enough signal for Smart Bidding to work well on the sale itself. Here it can be correct to keep a higher-volume secondary event as a bidding signal, provided it correlates with real inquiries and you check that correlation rather than assuming it.
Open the conversions table today and look at one column: which actions are primary. Then ask, for each of them, whether you would pay cash for that event happening. Everything that fails the test goes to secondary this afternoon. Expect the reported numbers to fall. That fall is not a loss. It is the first accurate reading the account has produced.