In 1 Cor. 9:27, Paul says:
But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified. (ESV)
What is meant by being "disqualified"?
This is a fascinating passage. In 1 Corinthians 9:24-27, Paul is using an athletic metaphor as both an example of the Christian life and a particular defense of his ministry. Within that context, we could consider this a reference either to losing (not winning) - verse 24, failing to compete effectively - verse 26, or to being rejected from competition. (Perhaps for breaking the rules?)
The Greek word translated "disqualified" is αδοκιμος (Strongs G0096), meaning "failing the test", "rejected" or "worthless". This word appears 8 times in the New Testament. In addition to "disqualified", the NET Bible also translates it "unfit", "useless" or "fail". This is also the word translated "depraved" or "debased" in Romans 1:28.
(My personal opinion is that it's one of the first two that I mentioned above, since there are already verses in the passage that refer to them. Also, the two concepts are very closely related - if you don't compete effectively, you can't win.)
See Hebrews 12:1-2 for another passage with a very similar metaphor.