偶然碰到

text: What is your plan, should you encounter porn on the Internet? What will you do if while innocently cruising the Net you "happen" upon an enticing Web site? The surefire way to avoid getting caught is to follow Joseph’s lead: Have a plan and follow it.

meaning: 

happen on/upon sth/sb

literary — phrasal verb with happen     /ˈhæp.ən/ verb [I]

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偶然碰到