I don't believe that questions regarding bugs are inappropriate in general, but there exists a line which separates answerable bug-related questions from bug-based speculations.
In the case of the Missingno. related question, the original question was "What's the bug on cinnabar island that can mess up your save?" This can be reduced into a more general category of "Bug avoidance". I.e., "How can I avoid bug X?" As luck would have it, people are familiar enough with the pokemon red|blue code to not only explain why it happens, but that information went above and beyond the original question.
Here's a (non-inclusive) list of what I think are "answerable" bug-related questions.
"How do I avoid/prevent bug X?"
"If I've triggered bug X, how do I
"I triggered bug X, what does this
mean for my game? / can I still do/get Y?"
All of these questions have definite answers, even if that answer might be "there's nothing you can do."
In contrast, the minecraft question that prompted this discussion was the following:
"I found this bug. Why does this bug happen?"
Said this way, the question seems like it might have an answer, but what I see the question as really asking is: "I found this bug. What developer oversight causes this bug to happen?"
There are a few main problems with this:
Irrelevancy of subject matter. The question is not really about the bug, or even the game, it's about the underlying code that runs the game-engine, and thus falls outside of our scope. I think a question like this might be valid on another sight, however...
Speculative nature of the question. Unless the developer themselves comes in to explain what went wrong with the code, there is no way to verify the correctness of any supplied answers. That means that any answers provided within the context of the site are speculative. Speculative questions should be closed, and indeed, already are.
I could see a case for asking this if the behavior that triggers the bug is unknown, but that would then fall into "bug avoidance / prevention", as outlined above, as the intent of the question is not why the bug behaves as it does, but how to prevent the bug from occurring in the first place. See: "Why am I paying for dirt?"
I might add more to this later. I'm having trouble thinking straight. (I blame the fumes from my roommates insecticides)