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

• Weimaraner
• Weinberg
• weir
• weird
• weird sister
• weirdie
• weirdly
• weirdness
• weirdo
• weirdy
• weisenheimer
• Weismann
• Weissbier
• Weisshorn
• Weizenbier
• Weizenbock
• Weizmann
• weka
• welch
• welcher
• welcome
• welcome mat
• welcome wagon
• welcomer
• welcoming