For example, you may see someone who you think needs help, and you think you know just what to do, but that may be more a reflection of where you (or better put, your ego) are coming from, what your perspective is, rather than what the person actually needs. And so by moving forward, you may often get a not very receptive response from the person you are trying to help, causing frustration if not anger.
Or, as unfortunately still happens at times with me, you are caught unaware and so your ego controls your response to a situation. You get defensive and lash out at the very person who you want to be there for. It is beyond painful to realize the power the ego continues to have even after so much spiritual growth.
The point is, you must have the intent on freeing yourself from the control of your ego mind, being in touch with your true Buddha self, and doing whatever is necessary to implement that intent. You will always have weak moments; we are all humans. But at least even if you are initially unaware, on reflection you will see very clearly what happened and it will be a learning experience, albeit a painful one. And you will be able to make amends.