The vast majority of pool players—both casual and regular—have one or more of their fundamentals out of whack to some degree.
The problem mostly arises because the player never took the time to check with someone who could show him/her how and why to do things correctly. The closest most people come to correct fundamentals is a flawed imitation of the friend who introduced them to playing pool. And that example probably had many bad habits.
If you are serious about becoming a decent player, here is a list of bad habits that can slow down your learning curve:
Grab the cue stick as close to the butt as possible, stretching your stick arm past your butt (yours—not the stick’s).
While butt grabbing, raise the angle of the stick as close to 45 degrees as humanly possible.
On the back stroke, wobble the stick side to side (or sometimes up and down), and then jab the stick forward “when it feels right.”
Swinging the stick is a gentle sideways curve—usually because your hip is in the way!
Angle your stick elbow away from your body while tucking the cue as close to your body as possible (aka chicken wing).
After you put your bridge hand on the table, you lean forward to put your head directly over your hand. (Some people think this is aggressive).
Put your stick hand as close to the half-way mark on the cue (on a jointed stick, this puts your stick hand right behind the joint).
Grip the stick very firmly so that when you stroke, the stick rises into the air (looking for a balloon to puncture?).
Alternate reason for firm stick grabbers—don’t want it to escape and fly across the room.
Use a a long, long bridge—at least 18 inches from your bridge hand to the cue ball—because you think it looks cool.
On every shot, you stand up during the shot—the famous “Jubilation” stroke.
Shoot every shot as hard as you can because when you make it, people respect you.
Move as many parts of your body as possible when making a shot.