I’d check out our post: Unusual Martial Art: Street Fighting to start giving you some ideas on the bad habits street fighters pick up. But, let’s go over some of the important pieces:
1) They’ll completely lack a base
A base is your footwork and stances, this is commonly the most boring and most ignored part of combat when you’re looking at movies and television. Unless the actor on the screen has been trained, they have terrible stances. Because your fighter has been training themselves, they’ll have been distracted entirely by the visual beauty of the staff work and completely ignore, or figure they already understand, the footwork.
As such, they won’t be able to take a hit from someone who knows what they’re doing and their attack power will be halved.
2) Bad hand position
Because they’ve been teaching themselves, they’ve never been taught how to hold a staff or how to transition their grip. Thus, their hands will constantly shift towards the center of the staff as they fight. They’ll even think this is preferable because it makes the staff go faster and appear closer to the speeds that they’re trying to mimick. However, the closer the hands get to the center point on the staff, then the more they lose control. This is untrue about all staff forms (like wushu where many techniques work off the center/balance point) but it will be true for someone who is untrained.
Even when it appears mostly right to the casual observer, the trained combatant will notice the points when control of the staff is completely lost like when it bounces off the fighters back when they are spinning it.
3) Basics are traded for flash
If your character is learning entirely from movies that involve actors who know what they’re doing with a staff, then it’s likely that they’re trying to imitate the flashier techniques. They’ll have a greater focus on spinning the weapon over smacking other people with it and probably show off by balancing the staff on their index finger (simple), bouncing it off their arms or shoulders (again, simple), and other simple manipulations that are easy to learn.
Since their understanding will be based almost entirely on movies and their own practice (with or without a partner) then their knowledge will be completely undercut because they lack a whole style and are making do with simple techniques.
They’ll also chain techniques more slowly and lack the ability to chain techniques together that they haven’t seen done before.
They won’t be able to fight other characters who know what they’re doing, though they may believe that they’re just as good (or better) than they are. However, they’ll also be very slow in comparison to those fighters.