If you have made a transfer and discover a small minor defect in the transferred pattern you don't necessarily have to wipe it down with Acetone and do it all over again, (although you should never get a failed transfer if you are within the parameters of the technique.)
There are several reason you could have a flaw... dust particle landed between the toner image on the paper and the copper surface, low density toner so there wasn't enough "plastic" in the toner to fuse properly or, the occasional hiccup from the printer itself. (Again, possibly from dust on the paper!)
Any good Sharpie® pen can correct a flaw, however, you can't just draw out the missing image. These waterproof pens have a solvent in them to make them waterproof and permanent. This solvent will dissolve toner! So how do we do it? Simple... it's in the way you use the pen.
You want to vertically "dab" the ink vs. "draw" the ink. Dabbing the ink puts a quantity on the copper and can then "melt" with the toner to maintain good 'resist' when you subsequently etch the board.
In order to "dab" the ink you have to get the tip a bit more wet than normal. So, with the cap on VERY TIGHT (!) snap the pen (black cap down) as if you were lowering the reading in a glass/mercury thermometer. This will wet the tip considerably. Remember this is permanent ink and if the cap comes off ink could be sent to the carpet!
Now remove the cap and touch the tip to the area(s) needed. If you need to draw a trace you can then put the tip in the wet area and the pen should then draw nicely. What you want to avoid is to have streaks in the trace. That will let the etchant through to pit the copper.