Jumping to conclusions
Jumping to conclusions (officially the jumping conclusion bias, often abbreviated as JTC, and also referred to as the inference-observation confusion) is a psychological term referring to a communication obstacle where one "judge[s] or decide[s] something without having all the facts; to reach unwarranted conclusions". In other words, it is when a person fails to distinguish between what they have observed first hand, and what they have only inferred or assumed.