Are you suggesting a commercial or other organisation would let someone sit on their flight deck without proper training?
No, they will have a type rating - otherwise they'd be breaking the law. There's a difference between a pilot that sails through his LST/LPC/OPC and one that struggles but still makes the grade. During a check the pilot can have any number of repeats at attempt one, fail up to 5 items at attempt one and still pass! Although TREs are acting on behalf of the CAA, I have heard of disparities with standards from one operator to the next.
So a pilot can bump scrape through his checks, having numerous repeats and retests and still keep their rating. The company for whatever reasons (poor management, costs etc) will keep the pilot on and then one day their skills are called into question and they fail. Unfortunately in the real world there is no reset button that allows you another shot.