The comment: Thanks, dad! Never again
Is a big spoiler for the puzzle in the question, I immediately made to flag for mod attention, but couldn't find whether spoiler comments/answers should be flagged?
There is no hard-and-fast rule about this, but people generally try to keep spoilers out of the comments, especially on a question (on an answer, I think it's fair to assume that if you're reading the comments, you've also read the answer).
The two strategies that are employed are either to comment vaguely enough that your comment can only be understood by someone who already knows the solution (e.g I think the third word should have the last two letters reversed), or use ROT13 to encrypt your comment so that it can't be read at a casual glance.
It doesn't hurt to flag the comment. I've deleted the comment in question here.