What is the proper way to introduce or expand relative context to a question, answer, or comment?

For example, A person asks for the best way to caulk the exterior of the pictured window sill.  A person gives an answer to the question (a sufficiently good answer).  I see that the window sill was constructed incorrectly. 

While the question was specific and the answer was correct for the question, I wonder what would be the proper way for me to bring up the issue of the sill, which neither the questioner or respondent has acknowledged is a problem, too?

Would explaining why the sill is a problem (and how to fix it) be an "Answer", a "Comment" to the question, or a "Comment" to the answer?

[Edit: Note, my inquiry is more to ask, "How to expand the initial scope of any question, answer, or comment by asking about or mentioning its context?"]

[2nd Edit (27.11.16): Okay, thanks.  I think I'd feel most comfortable doing a bit of both your suggestions: 

1) Edit the original question with an endnote that there may be an exception (in which the given answer wouldn't work/be best) or an issue being overlooked, 

2) Answer the question (if it hasn't been, yet) with a note about a possible exception or overlooked issue, 

3) open a new question (without waiting to see if the exception or overlooked issue takes on a life of its own)

I consider this question resolved, thank you for your feedback.]