Hotspur:
I would be interested to read members' comments regarding the use of an apostrophe in this post.
I don't think the apostrophe belongs there. At least, I've never seen it used that way. "Heads" isn't a contraction of anything, and I don't see any real sense of possession which would warrant an apostrophe. But even if an argument is made that there IS possession, "giving someone a heads up" is an idiom, and idioms often don't follow the rules of grammar, and do not always make sense. So, IMHO, no apostrophe.
However, it SHOULD probably be hyphenated - as in "heads-up" - b/c of the way Goodgulf used it in his original post. I googled "heads up" because Hotspur's question piqued my interest (and, b/c I probably have too much time on my hands), and found this comment by a reader: "When used as a noun or an adjective, there is a hyphen; when used as a verb or interjection, there isn't." I think I agree with this. So, when Goodgulf's character said "Next time give me a heads up," the character was using "heads-up" as a noun. If the character shouted "Heads up!" (as probably happened in many of the last story challenge entries, as a ball broke the window, or the vase, or dented the car, or broke all 3 items at once) there would be no hyphen.
As always, I acknowledge - doing my best Dennis Miller impression - that this is just my opinion; I could be wrong.
Now, back to procrastinating about other things instead of writing a new story.