I do everything I know how to letter a neat clean scroll, but it happens. I'm half done, or more, and I splatter the ink, leave out a text word, or some other goof.
The neatest way to handle this is to start over. The best way is to avoid making the mistake. Corrections, some very silly ones, are period, but they don't completely hide my error. At least I know when it's there.
What can I do to prevent mistakes? Here are the tricks I already use.
When I'm working I use scrap paper over the area I have yet to letter. I may also cover the area I plan to illuminate. This will avoid smears and splatters.
I keep my ink to the right because I am right handed. This cuts down reaching over my working area. I want my ink in a container I won't spill, handy, but not too close.
I don't eat in my studio either. And keep beverages in a covered cup.