Both amarcus and ProblemFactory are incorrect - the link only lists companies that were rejected by YC and went on to do well. Furthermore, as recently as a few years ago the terms were less fixed - YC would call to make companies an offer (although it was stated that the offer would usually be 7%). Even further before that, it was possible to negotiate with YC. A rumor (completely unconfirmed) that I heard somewhere on HN is that Sam Altman was able to negotiate Paul Graham down.
I personally met one company that turned down YC. They've since went on to raise more than $1.5MM. They thought YC wanted too much equity.