Problem 1 — Yahoo YST and Google Ads conversion tags firing duplicate events
This is the most common Japan-specific tracking error. When both Google Ads and Yahoo Japan Ads conversion tags are implemented on the same confirmation or thank-you page — as is standard practice for tracking purchases or form submissions — the same user action is counted as a conversion by both platforms. Neither platform applies cross-platform deduplication. Smart Bidding and Yahoo Japan's bidding algorithms each train against the full, unfiltered figure, producing inflated conversion volumes that make both platforms appear more efficient than they are.