Jump the gun
Have you jumped the gun before? Has anyone said that to you before? When you have jumped the gun, it means that you have done something early and prematurely.
A: Cindy looks unhappy this morning.
B: She must not finished her assignments last night.
A; Don't jump the gun to make that conclusion. She might just be tired.
In this example, A is tell B not to make the conclusion too early without understanding entire situation.