One of the biggest mistakes almost everybody makes in their workout happens during their warm up. Most people walk into the gym and immediately start doing static stretching, aka stretching without movement. The problem with that is that our muscles are elastic, they need a certain amount of blood flow to happen before the muscle is WARM enough to perform a static stretch. When you do static stretching without warming up first, you actually TEAR your muscles slightly instead of stretch them. This obviously hinders the workout you’re warming up for, and also slows your results; due to the fact that you are starting each workout with tons of tiny micro-tears in muscle fibers you were just stretching.
So never stretch unless you are already warm. The best way to warm up is to move; do some crawling, hanging, mobility, some light cardio, something that will get your heart rate up and increase your blood flow for at least 5-10 minutes. Static stretching is best left for post workout cool downs.