The most common analytics mistake is not a configuration error — it is timing. Organizations launch a website, run a campaign, or ship a product, and then install analytics to measure the results. By then, the data that would have been most useful is gone.
What you lose by waiting
Analytics cannot retroactively capture data. Without a baseline, you cannot measure whether traffic increased after a campaign or whether conversion rate improved. The analytics setup gets deprioritized during the build because it feels like something that can be added later. It cannot.
What to set up before launch
At minimum: install a web analytics tool, configure goal tracking for the primary conversion action, set up a search console account and submit the sitemap, and establish a baseline measurement period of thirty days.
The events that matter most
Configure tracking for the events connected to the site primary job first. If the site generates inquiries, track form submissions. If it drives calls, track phone number clicks. Everything else is secondary.