一不小心
verb C2- to be a little bit careless; to have a moment of inattentiveness; accidentally
From the library
只是每到这些时，他又常常喜欢拉上中国将来的命运之类的问题，一不小心，便连自己也以为是一个忧国的志士：人们是每苦于没有『自知之明』的。
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