Sunday morning, I opened my reports. CPL had spiked — but only on TikTok Germany. Google Germany, Facebook Germany, TikTok other countries: normal.
No Slack message. No heads-up. Just one number that didn't match the rest.
The flow was simple: ad → landing page → quiz start → lead → checkout → purchase. I broke it into two big steps instead of digging through every stage one by one — click-to-lead (everything that happens until the lead) and lead-to-purchase (everything after). This was before I had Claude for this kind of digging, so I kept it simple: two buckets, not ten.
Click-to-lead had dropped. Lead-to-purchase was fine. So whatever broke, it broke before the lead.
Now the drill-down inside click-to-lead. Landing page exit rate was normal — the same percentage of users clicked through. But quiz starts had dropped. Fewer people were beginning the funnel after leaving the landing page.
Quick sanity check: was I buying different traffic, or was the same traffic behaving differently? CPC hadn't moved. Same traffic, different behavior.
I dug into the funnel itself and found the actual bug: a settings change made over the weekend had routed some German traffic to the Portuguese version of the quiz instead of the German one. Users hit the German landing page, clicked through as usual, then landed on a quiz they couldn't read. They dropped off before starting.
Quiz completion rate for the users who did start was unchanged. The drop was entirely at the entry point.
I fixed it the same day I found it. CPL took another day or two to fully settle.
I found all of this by breaking down the numbers, not by being told.
Two steps first — click-to-lead, lead-to-purchase — so you know which half broke. Then look inside that half, stage by stage, to find exactly where users are dropping. Then a quick CPC check to confirm you're comparing the same traffic before you go blame behavior.
Where, then what stage, then whether it's traffic or behavior. In that order.
When CAC spikes on a weekend, what's the first split you check?
Originally published in The Growth System newsletter on LinkedIn.