1. By accident
   - haply [archaic], by luck 
2. Without advance planning
   - accidentally, circumstantially, unexpectedly 
3. [archaic] Through chance, "To sleep, perchance to dream.."
   - perchance [archaic]

Encyclopedia: By chance

• vat dye
• vatic
• vatical
• Vatican
• Vatican City
• Vatican Council
• Vatican I
• Vatican II
• Vatican Palace
• Vaticanism
• vaticinate
• vaticination
• vaticinator
• vaudeville
• vaudeville theater
• vaudeville theatre
• vaudevillian
• Vaudois
• Vaughan
• Vaughan Williams
• vault
• vault of heaven
• vaulted
• vaulter
• vaulting