Spelling. These are just dumb little things to look for. Did you spell all the identifiers the way you think you did. The problem is, when you read code you see what you expect to see.

拼写错误，你可能得找找这些粗心的小错误,你是否拼对了标识符,问题是当你阅读代码的时候,你看到的都是你以为的。

麻省理工公开课 - 计算机科学及编程导论课程节选