Chris Horner was competitive in races when he was 41. That was unheard of in modern bicycle racing until Valverde, a convicted doper now racing after his suspension, continued to race. This is all after the invention of EPO so that could be a coincidence or not.

In the redacted documents from the USADA investigation of Lance Armstrong, several people are not identified still, rider 15 is never identified but his name is redacted out. Because they used a font that is not uniform width per character, there has been a lot of speculation over the years that rider 15 is Chris Horner because a.) his name fits the redacted space, and the parts of the characters USADA did not redact match the letters in Chris Horner, b.) the background described for rider 15 match Chris Horner.

Many people came to this conclusion separately. Someone who actually didn't dope and raced against Horner in his first prime wrote a lot about it - a sample http://stevetilford.com/2013/10/24/unbelievable-chris-horner...