For example, power outage occurs at the same time the UPS batteries are being changed. Bypass fails and Diesel generators fail to kick in. Circuit breakers blow everywhere making it extremely difficult to get the generators back on line. This all happens in the middle of the night in a winter storm (or 'lightning storm) which causes a further delay in response time.
Been there, done that.
Edit: Also, bureaucracy, lack of documentation, and a manager CF added several hours to the outage. Sometimes you just have to STFU and let the geeks fix the problem.