Chrissy (mother): What's wrong Billy?
Billy (Chrissy's 3-year old son): I fell down.
Chrissy: Awww! I see you've got a little boo-boo.
Joe: Ouch! I just pinched my finger in my car door!
Sam: Oh no! Did you get a little boo-boo? You want me to kiss it? (totally joking with his friend)
Meaning: "a boo-boo" is a child's word for an injury. It could be a cut, a scrape, a bruise, etc. This word is only used with speaking with small children. It is not used when speaking to adults. For this reason, context #2 is two guys joking with each other. Same is teasing Joe, as if he were a little boy, or weak.
'The server made a boo-boo! We couldn't use our computer for hours '
'Josh gave you the wrong file, he made a boo-boo'
Meaning: A mistake
'I love you BooBoo, miss you already'
Meaning: Someone dear to your heart: your babe/baby, your Boyfriend/Girlfriend. It is used for someone/something who needs to be kissed and cuddled every second of the day.