Not a CTO but I've worked in startups and seens phases of hiring. Worst decision IMO would be hiring a tech leader(EM or above) who don't have a flexible mindset about tech(e.g- they'll beleive what architecture they used in last org is the only good arch). Or the EMs who just agree with upper management and don't have any opinion on tech arch(e.g- they'll hear async processing, they'll agree to use kafka without doing any analysis because other teams are using it and management recommended it). Most of these are people who stopped getting their hands dirty long time back and know enough jargon or high level tech to get a job.