Related Clues:

• Out of sync
• Quite strange
• Unpaired
• Missing a few marbles
• Out of tune
• Kind of duck
• Like all primes but one
• ''They're ---!''
• Lacking a match
• Atypical
• Switch setting
• Quite unusual
• Unmatched, as socks
• Out of the ordinary
• Like Oscar and Felix's pairing