come across sth./sbdy.（尤指偶然）遇到，碰上，发现eg:She came across some old letters in the courseof her search.她在找东西时偶然发现了一些旧信件。

run across sth./sbdy.与……不期而遇，偶然碰上（尤指令人愉快的人或事物）eg:I ran across an old friend in the street.我在马路上偶然碰见了一个老朋友。