1- Make the assumption that you know what the problem is without seeing what is actually happening
2- Make the assumption that you know how to fix a problem without finding out what is the real cause of it
3- Make the assumption that the activities you have done to fix a problem are working without checking to see if they are actually working and doing what you were expecting.
These 3 mistakes have the same issue in common, which is you are not going to the gemba to see and Grasp the situation. Remember that there is no better way to understand a situation that by going and see what is actually happening. This is done not only to understand the reasons or causes of a problem, but also to see and confirm that things are happening as they are supposed to work.