I've heard PG complain about using "YC" in titles (from guidelines: "In titles, please don't describe things by their relation to YC unless they're actually associated with YC.") But I didn't think the use of the word "reject" was the issue. YC even uses that word themselves in their "non-judgement" letter. ;)

"Groups we rejected will go on to create successful startups. If you do, we'd appreciate it if you'd send us an email telling us about it; we want to learn from our mistakes."

So they are also apparently are interested in hearing about such things too.