A conversion that never fires, or fires on everything
This is the most consequential and the least noticed, because a broken conversion tag produces a plausible-looking dashboard rather than an error. Two failure modes dominate: the tag never fires, so bidding has no signal and the account looks like it has no demand, or the tag fires on every page load, so the algorithm learns that all traffic converts and optimises towards volume. Verify by submitting the form yourself and confirming the event arrives. We have found production sites where no conversion had ever been recorded because the tracking function was never actually attached to the page.