A slip of the pen:
A minor mistake in writing.
意思是 “笔误”，指 “（书写时的）拼写、语法或用词错误”。如果某人 “make a slip of the pen”，意思就是此人 “因疏忽而出笔误或写错字”。slip意为打滑，恰如中文里经常说的“手滑”写错了，这里把手滑的责任怪给了笔，笔滑写错了:P。
例句
I made a slip of the pen and wrote that the car was worth £1,000 not £10,000!
我笔误写错了，这辆车值一万英镑，而不是一千英镑。
Due to a slip of the pen, I misspelt the name of the bride on the wedding invitations.
由于笔误，我在婚礼请柬上拼写错了新娘的名字。
One slip of the pen and you could lose valuable points in your exam paper.
在考试中出现笔误，你就可能丢掉宝贵的分数。
