Easily the most frustrating and common mistakes that I see shooters make revolve around an improper and/or inefficient grip. We have covered at length the advantages that a biometrically correct and ergonomically efficient proper grip will provide you, but we haven't really provided insight as to the common mistakes that people make. Let's take a look at some of these grips, and why they are insufficient and sometimes downright dangerous.
This is probably the most common mistake that I see in regards to an incorrect grip. This is when the shooter grabs the grip frame of the handgun with their shooting hand, and has their thumb diagonally draped across the panel where their support hand should go. Inherently, this prevents the "meaty" part of the palm on the support hand from making maximum contact with the grip. This provides a path of lesser resistance on the side of the support hands, which results in a movement upwards and towards the support hand side during recoil. This negatively impacts the speed of front sight acquisition during the follow-up shots. Furthermore, since we don't have maximum contact with the weapon- we also don't have maximum recoil control. To compensate for this, we are left with the poor option of simply squeezing our hands harder. This can result in hand tremors which will negatively impact sight picture. Moreovoer, since you are having to exert more muscle and less technique to control the firearm, you will have a harder time gripping the gun consistently with this method. This means that you will have trouble producing repeatable results in regards to your point of aim/point of impact. In short, your grip sucks. Fix it!
An outdated yet not dead grip brought about during the days of the snub-nose revolver's hayday, the tea-cup grip has been a long time Hollywood favorite. Basically, the shooter will place their support hand underneath the magwell and hold the firearm and shooting hand as if they were a "tea-cup" in a futile attempt to add stability to the gun. While I guess one could argue that this grip is more stable than shooting one handed, I would argue that it's only slightly better. You aren't doing yourself any favors in regards to recoil control or providing faster follow-up shots, and you aren't providing any advantage to basic weapon manipulations. In short, your grip sucks. Fix it!