One very interesting example I know is Algolia. They first applied when they were very early stage, in 2013 if I'm correct. They got rejected mainly because of their poor ability to track user metrics. Then they re-applied in 2014 and got selected for the Winter batch. Last year they raised $18M after an epic roadshow with all key VCs sending termsheets. My opinion is that YC strongly values your ability to be focused, execute and track/analyze data instead of technology and product/market fit - especially in early stages.