People with dementia are no different to you and me in our desire to do the best we can so we can feel good about ourselves and our efforts.
However, they like us, often make mistakes. These mistakes are more frequent and more obvious than our mistakes and we can recover from them and disguise them better than a person who may have limited verbal ability. Gaffs in conversation, forgetting things we know we know, and mixing up how we should behave in public can cause social embarrassment to the person themselves and to others.
If we correct and point out the mistakes that the person with dementia makes (often out of our own embarrassment) we may make the situation worse by angering the person, and embarrassing or humiliating them.
So what should you do? Firstly, take their perspective. What does it feel like to make mistakes? It feels bad. We turn the anger we feel in on ourselves and attack ourselves, often at an unconscious level, making us more angry and irritable and reactive to the criticism of others. This is no different to people with dementia. Same mechanism at work. Once you can identify with their likely feelings, work to protect them emotionally and draw them, invite them to the appropriate response or solution. It may take a few more seconds but it is worth the time now rather than the ten minutes you will have to spend calming them down when they are angry.
Second, be clear about the issues. If there is no physical safety issue you may not have to react immediately. Take your time to think about how to respond so that you can be calm and reasonable.
Third, use your non-verbal body language to communicate acceptance, affection and respect to the person. Smile, make eye contact, use open hands fingers down, use inviting gestures, move at a relaxed pace.
Fourth, do something familiar together that is cognitively easy and no challenge to the person.
Fifth, use your knowledge of the person to find ways to connect and re-establish rapport if the person becomes embarrassed or angry. You know how best to make them feel calm quickly.
best of luck in your caring.