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

• large-scale
• large-toothed aspen
• largehearted
• largeleaf holly
• largely
• largemouth
• largemouth bass
• largemouth black bass
• largemouthed bass
• largemouthed black bass
• largeness
• larger
• larger-than-life
• largess
• flux
• flux applicator
• flux density
• flux density unit
• flux unit
• fluxing lime
• fluxion
• fluxional
• fluxmeter
• fly
• fly agaric