What is dumbest thing you've done to disqualify yourself in the interview process?

I went through their hiring process (5 steps) after being contacted by their internal recruiter. I went all out in my pre final interview research. Reached out to a few former reps to learn more about the company and role, was more prepared than I ever have been for an interview.

I CRUSHED the interview. At the end, both VP and director were blown away and I had to await a final "culture fit" meeting with CEO before receiving official offer.

3 days later received the automated reject email so I followed up with Recruiter to find out why. Director replied back and gave me time on phone to explain that it came down to me and one other candidate and the deciding factor was me not sending a follow up "thank you for the interview" summary style email. That was it. A simple fucking email and I would be making almost 2x what I make now.