> Excellent interview
to turn into
> couldn't even handle explorer.exe.
I had to deal with a similar situation. The size and scale of my employer at the time meant the employee stuck around for multiple years. They never got worse but sure never got better.
In our case, the problem was that our interview questions were based on algorithmic knowledge with some OS fundamentals. This person happened to be really good at those types of problems. But ask them to change a few lines of Python or Javascript and they just could not figure it out.
So the underlying flaw was assuming that anyone who could figure out algorithmic optimization in C++ (psuedo-code, we weren't looking for perfect syntax) under interview pressure would also figure out higher level languages, test frameworks, CI systems, etc, in an efficient fashion.
My simple solution was to push for a more diverse set of interview questions and interviewers. Actually, a "take home" style problem might have fleshed out these weaknesses but that was too radical, relative to the company culture, to even propose.