When we did annual pen testing audits for my last company, the security audit company always offered to do phishing or social engineering attacks, but advised against it because they said it worked every single time. One of the most memorable things they shared is they'd throw USB sticks in the parking lot of the company they were pentesting and somebody would always put the thing into a workstation to see what as on…
One of my favorite quotes is from an unnamed architect of the plan in a 2012 article about Stuxnet/the cyber attacks on Iran's nuclear program:
"It turns out there is always an idiot around who doesn't think much about the thumb drive in their hand."