Q: Have there been any startups you've later regretted rejecting from YCombinator? A: Sure, several. But I can't name names because it's not for me to disclose that they applied.
Additionally they also mention about contacting them on rejected applications[2]:
"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"
Here are some that are known to have been rejected by YC and have gone onto raise funding (which is not a perfect metric by any means since funding ≠ success) or have been acquired:
- SendGrid - http://sendgrid.com/ - Raised $27.4M[3]
- CouchOne - http://www.couchbase.com/ - Raised $56M[4]
- AfterTheDeadline - http://afterthedeadline.com/ - Acquired By Automattic
- LightSail Energy - http://lightsailenergy.com/ - Raised $42.8M[5]
- SignPost - http://www.signpost.com/ - Raised $15M[6]
- Storenvy - http://www.storenvy.com - although they were kicked out of YC[7] Raised $6.5M [8]
[1] https://web.archive.org/web/20120814111130/http://askolo.com...
[2] http://ycuniverse.com/yc-applying-interviewees
[3] http://www.crunchbase.com/company/sendgrid
[4] http://www.crunchbase.com/company/couchbase
[5] http://www.crunchbase.com/company/lightsail-energy
[6] http://www.crunchbase.com/company/signpost
[7] http://joncrawford.com/post/20378314843/how-i-got-kicked-out...
[8] http://techcrunch.com/2013/02/12/storenvy-goes-from-getting-...