I had one egg that hadn't pipped that I opened, she's doing fine now.
It was the first egg of my last group to start cheeping. The next day all the others had pipped and most had zipped and a couple had hatched I noticed that one egg still hadn't pipped. I tapped and listened and it was still peeping. Left it a few hours longer and checked again, less peeping. Later, no peeping, but still moving... couldn't wait any longer. I took an exacto knife and carefully scratched a small hole in the middle of the air cell. Followed the directions here and eventually wound up removing most of the shell.
She's the smallest of the lot, but vigorous and healthy at 10 days. I'm so glad I didn't let her go, even though it was nerve-wracking. Good luck whatever your choice.