Peter Principle
The idea that employees rise to the rank just beyond their competency, as they are evaluated on performance to their current role and not their intended one — at which point they cease to be promoted.
Origin
Canadian educator Laurence J. Peter formulated the principle while teaching at the University of Southern California. He and journalist Raymond Hull published it in their 1969 book The Peter Principle: Why Things Always Go Wrong, which — after being rejected by over a dozen publishers — became an immediate bestseller, eventually selling eight million copies in 38 languages.
Everyday Use
This is a good term for someone who works around coworkers everyday -- a reminder of its use hardly seems necessary! Yet this reminds us that no system is perfect -- almost everything we interact with, be it a coworker, or software, or process, or tool -- is 100% perfect, or even 100% for that job.