“Let's not jump to conclusions.”
You're on vacation with your wife. Your wife can't find her earrings. She thinks that they might have been stolen by the cleaning staff. You don't want to accuse anyone of stealing until you're sure that the earrings are gone, so you say this.
Let's not jump to conclusions.
"Jumping to conclusions" means accepting an explanation too quickly. For example, if a coworker is late for work, you might think that she overslept. However, this would be "jumping to conclusions" because there are other possible explanations for why she's late: she might have gotten in a car accident, for example.
"Jumping to conclusions" is a bad habit. People often warn each other not to do it:
Don't jump to conclusions.
You can say "Let's not ___" as a suggestion. For example:
Let's not go out tonight.
But another way to use "Let's not" is as a kind of warning or criticism. For example, when two people are arguing, you can say something like this:
Guys, let's not fight.