Digital painting, which I prefer, allows for the omnipotent Undo command. Real painting does not allow that.
There are two ways of correcting mistakes. The first is to take a clean brush loaded with water and try to mop out the error before it dries. A wet fingertip will work if the mistake is fresh.
If that doesn't work, a mistake must be painted over, disguised, or cleverly incorporated into a new design. There's a story about an artist who made the mistake of wearing his best formal shirt while touching up the piece he was going to present at a party: he splashed indelible ink on one of his pristine cuffs. Rather than try to wash the ink out - there was no time anyways - he took a pen and drew damask designs on both of his cuffs and his collar to cover up the stain. The artwork was well-received and his shirt was the hit of the party. It's like a Mentos commercial!