I've worked with people like this, and it's still hard to believe with evidence. I've changed my interview style from asking questions about programming to one that has candidates do a small design, then write one or two small programs implementing the design they just did. A surprising number of candidates are able to complete the design part well and then unable to make any real progress on the code. Most of them were phone screened before my interview; some had multi year careers in the industry. This feels different than people who are nervous or otherwise having a bad day, as they are exuding confidence while simultaneously being unable to write a while loop.