That's how this happens!
1. Don't panic.
2. If going for an attempted rescue, write the correct letter in place, then once everything's really dry, only the "wrong" strokes need removing.
3. Learn from your mistake.
Anyway, this piece continued to grow (it's very large), but in the end other factors meant this version got abandoned - more on that in another blog entry, I feel. So the right thing to do was to rule up and start again.
Surely I wouldn't make the same mistake again.....?
Oh no - I managed to make it WORSE!
(And tell you all the other tales of woe, and maybe even show you a finished piece!)