When we make a mistake while writing in pen, we usually scribble out the mistake. But whether you draw zigzags, spirals or scratch left and right, it is difficult to complete hide the mistake as the letters will show through the scribble. The brain has a fantastic skill of recognising letters, so it can read between the lines, so to speak.
To truly obscure your mistake or to redact confidential information, the best way to scribble it out is by writing on it. If you write over your mistake repeatedly with random letters of the alphabet, it will completely obscure whatever word lies beneath.