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

• Equus zebra zebra
• er
• era
• eradicable
• eradicate
• eradication
• eradicator
• Eragrostic abyssinica
• Eragrostis
• Eragrostis curvula
• Eragrostis tef
• Eranthis
• Eranthis hyemalis
• erasable
• erasable programmable read-only memory
• erase
• eraser
• Erasmian
• Erasmus
• Erastianism
• erasure
• Erato
• Eratosthenes
• Erb's palsy
• Erb-Duchenne paralysis