No hypothesis. “Let's try a different headline” isn't a hypothesis. If you don't know why a change should work, you can't explain the result or repeat the win.
Stopping at the first good result. Variant B is up 30% after week one? Wait for the planned date. Small samples are unstable, and the novelty effect fades.
Testing on untargeted traffic. If 60% of visitors are bots or come from irrelevant searches, the result is distorted. Clean up traffic first.
Ignoring segments — or fishing for them. An average can hide differences between new and returning visitors or mobile and desktop. But plan one or two segments in advance; hunt after the fact and you'll eventually “find” a random winner.
No link to the CRM. More leads doesn't mean more revenue. Without a variant label in the CRM, you won't know what happened after the form.
A test that runs for months. The market shifts and the result loses meaning; Google also advises against running tests longer than necessary. To stay safe in search, use rel="canonical" on variant URLs and temporary 302 redirects, and never show crawlers a different version of the page (
Google Search Central guidelines).