Here's what I started doing that works for me: I take care of the mistake as if it was an appointment or task that I have competed. For me, that means I check it completed and highlight it and then I make sure I write it again on the correct date. Sure I know that it was a mistake, but seeing it follow the same layout of all my other tasks makes it not bug me as much.
How do you cover up mistakes in your planner? Please share them with us in the comments below.