polygraph false positives...
It's also easy to fail them. When I was 19, I applied for a job at a convenience store... circa 1982... and was forced to take a polygraph. I failed miserably... I have no idea why. They got a false positive and I got fired ( I had been working there about three weeks).
Many years later... like 10!... someone called from their home office and wanted me to sign something related to that polygraph. They were doing some kind of CYA about there whole history of polygraphs. I was just thankful that they fired me and I moved onto something more lucrative.