A still tongue keeps a wise head

Prudent and intelligent people usually keep quiet and don't do much talking. 

”He was so quiet in class I thought he did not understand the lesson. But he got the highest grade in the exams. Indeed, a still tongue keeps a wise head!”

A steal

This is something that costs much less than it is really worth.  
“I got this jacket for a steal, at half its original price!”


 

A square peg in a round hole

He is one who finds difficulty blending, or who cannot seem to blend with the people around him; a misfit. 

“Everybody else in his class loves outdoor activities but all he wants to do during free time is stay in his room or sit in front of his computer.  He’s a square peg in a round hole.”

A slip of the tongue

This is a careless remark which reveals an information not intended for disclosure.

”He had intended to keep their defense strategy a secret but a slip of the tongue gave them away.”

A slap on the wrist

This refers to a punishment that is much lighter than what the rules or the law prescribes for the offense committed; a mild punishment. 

“The young man should be charged for serious traffic violations but his famous father intervened so he got off with just a slap on the wrist!”

A sitting duck

This is a person who is an easy target of criticism or attack. 

“The superstar is very vocal about his opposition to same-sex marriage. He is a sitting duck for the LGBT community.”

A rude awakening

This is an unpleasant situation or event that makes a person realize and accept the truth.

“For a long time I did not believe the news about our sinking economy.  The rude awakening came after I lost my job and my home.”