Your friend was using your camera and accidentally deleted all of the photos that were saved on it. You don't want to seem too angry, so you say:
It's not the end of the world.
disappointment, mistake, set phrase, understatement 
At work, you sent an email message to someone in the tech support department to get one of your computer programs fixed. The tech support manager sent you back an angry e-mail saying that you were supposed to come to him to help, not to the other person. You didn't know, but now you're annoyed that the manager got angry at you for such a small mistake. You're complaining to another coworker about it. You say:
Jeez! It was an honest mistake!
annoyed, complaining, gossip, interjection, mistake, work 
Today your daughter made a really funny mistake on her science homework. You wanted to laugh about it, but you didn't want to upset her so you stopped yourself from laughing. Now you're telling your husband about it and explaining why you did that. You say:
I didn't want to make her feel self-conscious.
children (topic), emotions, homework, mistake, simple past, worry 
Your daughter made a big mistake on her science homework. It was a really funny mistake but you didn't want to make her feel bad by laughing at her, so you tried to control yourself. Later, you tell your husband about it. You say:
I could barely keep a straight face.
children (topic), difficulty, laughing, mistake, 한글번역 
You wrote a book, and sent it out to lots of different publishers. One publisher agreed to print it, but said that you had to pay $4,000 to cover publishing fees. You were excited to have your book published so you gave them the money, but now you've only sold a few copies of your book. You regret paying the company, so you tell your friend:
In retrospect, I probably should have known it was a scam.
You sent out an informal e-mail to a group of friends telling them about a camping trip you want to invite them all on. After you sent it, you realized that you wrote the wrong date. You send out another email to correct the mistake:
Oops! I meant 2 weeks from yesterday, not today.
correction, mistake, time, 日本語サポート You're telling a story about a silly mistake you made. You thought you couldn't get into your house, so you spent the night in a cheap hotel. Now you're telling the punchline of the story. You say:
It turns out that my roommate had left his window unlocked anyway.
mistake, past perfect, punchline, roommate, storytelling, 日本語サポート You're practicing the guitar. You're trying to play a song while your friend listens, but every time you play it you get one particular part of it wrong. You say:
I keep making the same mistake over and over.
Earlier today, you meant to call one person, but you called someone else instead. Now you're telling your friend about it. You say:
I got mixed up and called my landlord by mistake.
get, mistake, phone, phrasal verb, simple past You wanted to call your friend, but instead you called your landlord's phone number instead by mistake. You know your landlord pretty well. When she answers the phone, you say:
Oh, hey Debbie. Sorry, I meant to call someone else.
interjection, mistake, phone, stative verb, telephone 