Is it possible to promote an employee beyond his abilities? Absolutely. Consider the following scenario:
Joe has long been a loyal and hardworking salesman for ABC Widgets. In fact he's such a charismatic and passionate fellow that when the monthly tallies are conducted he's regularly the top salesman for the period. Yes, he could sell ice to an Eskimo.
A new sales manager position opens up within the company, and immediately the company owner considers Joe for the position. After all, who would be a better manager for the company's sales crew than his star salesman? An ace salesman could easily train others how to perform the job well and as such would be the natural choice for a promotion, right?
Wrong!
Let's take a look at what Marcus Buckingham and Curt Coffman, authors of the fantastic book First, Break All The Rules, have to say about such situations:
"The first fallacy is that each rung on the ladder represents a slightly more complex version of the previous rung. Consequently, if a person excelled on one rung on the ladder, it is a sure sign that with just a little more training, he will be able to repeat his success on the rung above. The best managers reject this. They know that one rung doesn't necessarily lead to another."
Now let's head back to the example I laid out above. Joe is a perfect salesman, but it would be a serious mistake to think a perfect salesman would automatically make a perfect manager for a sales crew. Why? Sales and management/training are entirely different beasts! While it's possible to find someone that excels in both, in most cases each job must be treated separately.
For many years I've worked in management and training. I've also done some sales presentations, but although I know how to do sales, if I was given a sales position I would likely fail miserably. I'm not a natural salesman at heart, and if I were forced to perform such a job day-in and day-out my morale would take a crash. I consider myself a hard worker, but I'd never excel to Joe's level in the example above.
The same could be true for Joe. Salesmen often love the "thrill of the hunt" and being exposed to so many new people during their sales calls. Management, on the other hand, requires working with the same people regularly. Instead of chatting it up with prospects during sales calls, as a sales manager Joe would be doing things like performance reviews of those under him, training newcomers to the team, acting as a moderator when employee arguments ensue, paperwork, etc.
In other words, a sales manager position is not the same as a salesman position, even though on the surface the manager position would seem like a natural extension of the former. If Joe doesn't have a deep understanding of people, he won't be able to keep his team motivated properly. If he doesn't have a deep love of the job, he'll soon grow bored with training others to do the job he once so loved. If he's not extremely detail-oriented, he might not be able to perform accurate evaluations or dot all the "I's" and cross all his "t's" on his paperwork.
Never make the mistake of assuming someone highly qualified for one position will be highly qualified for another. Analyze the skill sets required for each position carefully and put aside any feelings of good will you have for a particular employee. You need to be completely neutral and impartial when you consider a candidate for a promotion, because the best employee in the world can turn into a disaster when mismatched.
As a side note, if you work in human resources or management, or have an interest in those subjects, I would strongly encourage the abovementioned book. First, Break All The Rules does a wonderful job exposing the weaknesses inherent in traditional corporate ladders and management practices, citing case examples and empirical evidence to show why flexible and freethinking managers are the most successful ones.