When you assume bad intentions, you believe something about the person who made the mistake that is rarely true. When you treat mistakes like they are intentional, you are treating the person who made that mistake unfairly. You are accusing them of something of which they are not guilty.
When you assume bad intentions and punish the person who made the mistake, you may get fewer mistakes. You also get an employee who is afraid to take initiative and unwilling to use their own resourcefulness to take independent action and make decisions. This is how you manufacture engaged employees who end up being dependents. You are creating employees who wait for your permission to do the job you hired them to do.