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

• bimillenary
• bimillenial
• bimillennium
• bimli
• bimli hemp
• bimodal
• bimolecular
• bimonthly
• bimorphemic
• bimotored
• bin
• bin Laden
• bin liner
• binal
• binary
• binary arithmetic operation
• binary code
• binary compound
• binary digit
• binary file
• binary notation
• binary number system
• binary numeration system
• binary operation
• binary program