Sometimes there are very short anwers for real questions. In the latest example, a working solution was given in a one-liner comment. Now what is the policy/advisable, posting that as an answer which then can be accepted (or at least the question isn't unanswered anymore), or leaving it as is?
In this case, I would take a moment to explain exactly why this fixed it, maybe some sort of 'brief lesson on the importance of only joining to what you need'.
By taking a moment to type just a brief little couple of sentences, you can turn what was merely a few words response into something that seems to have quite a bit of substance to it. And you've probably reiterated something someone else already knew, but a refresher has never hurt anyone. It's why guys like GBN still read answers from guys like AaronBertrand, even though they obviously both know what they're doing. Making sure they stay sharp. Oh, and to answer your question:
It happens, it's ok.
My philosophy has been: if I've added a comment (whether short or simple) and it ends up being the answer, and the OP tells me "thanks, that solved my problem!" I'll convert it to an answer. But I agree that it can help make it more substantial if you add some fluff, context, an example, etc. And I usually do that. Here is an SO example from today - my initial comment was something like "Have you tried moving the .BAK file to a folder where SQL Server has access, such as the backup or data folder?" The OP replied, told me that solved the problem, so I converted it to an answer instead.
The nice thing about converting a comment that happens to be the solution is that future readers might not see the comment otherwise. Also if a question is answered by a comment, it remains in the unanswered queue even though a valid answer exists (regardless of how short the "answer" is).
A counter-example would be where the answer is a one-word answer by definition. Like what function is used to convert a string to lower case. In that case (no pun intended) the problem is the question; it really should be closed as RTFM.