I'll get the obvious "I don't really buy this but it's funny" out of the way and move straight to "Something he didn't have last time." "He" is Voldemort. That line's from the bit of OotP when they all think Voldemort has some sort of weapon. Actually I'm pretty sure the next line is basically, "Like, a weapon?" And yeah, it might get repeated a couple of times. But it's clearly not one of JKR's most common sentences, especially if "he shrugged" etc is included for Twilight. Anyway, this overanalysis is revenge for your overanalysis of Parkour-Legolas.
This quick-and-dirty script says the most common sentences in the Harry Potter series, ignoring marketing bullshit in the ebooks I grabbed off usenet, are "Harry!" (15 occurrences), "'Yeah,' said Harry." (9 occurrences), "Clunk." (9 occurrences), "'No,', said Harry." (9 occurrences), "Harry?" (8 occurrences), "'Yes,' said Harry." (7 occurrences), "Good." (6 occurrences), "Yes." (6 occurrences), "No!" (6 occurrences), and "Crack!" (6 occurrences).
I did this while waiting for coffee to brew, though, so take it with a shaker of salt."I don't really buy this but it's funny"
Your comment made me realize a) how long it's been since I've read Harry Potter and b) how little of the English names I recognize.
The lesson here, more likely, is that even though these novels are long, it is rare for the same sentence to be repeated exactly multiple times. It is also highly likely that the sentences that are repeated are extremely short because the longer a sentence is, the less likely it will be repeated (unless deliberately). In addition, the repeated phrase might appear heavily in one portion of the book and only be repeated a few times after that, if at all. However, the deliberate reiteration is enough to cement the phrase in a list like this. So eightbitsamurai, not that I've read hunger games or care to, I wouldn't be surprised if the introducrions of katniss to all the other characters and game contestants might include a repeated query, "how old are you?" Certainly I expect it would come up when she is being interviewed etc. It doesn't surprise me that all these sentences seem very banal because you wouldn't really want to repeat anything interesting a ton of times, it would get boring by the 4th time you read it.
I found this surprising, too. Sirius says it in OotP when he's telling Harry about the Order, and then Harry repeats it in several DA meetings and in his thoughts throughout the book. Still, I find that hard to believe that it's one of the most common sentences. I suppose the phrases that we'd expect to find in the list are parts of longer sentences or slightly varied each time (?).... I also find it hilarious that "Harry blinked" made it onto this list, but it kinda makes sense