Starting a demanding sport like Brazilian jiu-jitsu is very difficult. Regardless of talent, most of us have been guilty of making mistakes, some even repeatedly!
Jiu-Jitsu is all about the flow. If the opponent knows what you have in store they will have an easy time defending 99 times out of a 100. This is why high level grappling is so amazing to watch, every move has a counter move!
Avoid shoe-boxing yourself. Common mistakes of this type are from beginners who go for cross chokes before even trying to pass half guard, going all in for the armbar or insisting on a arm triangle all the way from half guard.
A very common mistake that is often done by Wrestlers is to grab the far single leg in BJJ. This is a common control in Wrestling but a terrible idea in BJJ. Jean Jacques Machado shows a wicked counter that you can do when they do that:
