Now usually the Obliviate charm wipes whatever memories you choose and that's it, however, with a bit of skill in legilimency you can actually use it to tweak memories instead, planting a fictional event into someone's brain. Even worse is that stronger wizards could theoretically rewrite your entire life, building up a fictional existence for you that you believe with all your heart, everything you were, every shred of your personality erased with a single word. I do this to a lesser extent with Umbridge, I make her believe that she reprimanded me for my speaking out of turn and that she went away to research my claims and conceded that she will need to teach the practicals.