Since the last posts I decided to correct it with the suggestion of @jamesalexandermcmillan

It worked well, only a thin line can be visible from the sides, but it is almost invisible.

I think using veneer is the best option.

Or:

cut the next slot where it should be then remove wood between the slots with a router to the depth of the slots and patch the gap with a piece of ebony and epoxy and sand the patch to the shape of the fingerboard. The frets will cover the change of wood. A join will be visible from the sides though. 

Yeah, this is feasible and a reasonable solution to the problem. Any of these solutions are a bit fiddly and time consuming. 

I think in this case, using a new fretboard is certainly the easiest choice. In other cases it may not be the best choice because of things like cost (budget) and if the fretboard was already glued onto the neck. It’s lucky (in this case) that it isn’t already glued to the neck. 

This is a good contribution to the conversation @jamesalexandermcmillan 👍

cut the next slot where it should be then remove wood between the slots with a router to the depth of the slots and patch the gap with a piece of ebony and epoxy and sand the patch to the shape of the fingerboard. The frets will cover the change of wood. A join will be visible from the sides though. 
I’m not a qualified doctor though!

jim

Cut the fingerboard all the way through the misplaced slot, lay it on a sheet of wax paper, and glue the two halves together, then proceed as it were a single piece.

I am not sure that I would recommend doing this as the fretboard is such a key, structural, part of the neck (even though it is in compression once the instrument is strung) that having it made up of two pieces sounds like a bit of a risk. There is so much other time and money that goes into a build I don't see the point in compromising an entire instrument for such a relatively low cost piece piece of material.

Do you recommend me a way to "recycle" the first fretboard.

When something similar happened to me I used the damaged fretboard to repair the mistakes I made in the next one! 🤣 🤣 🤣 

Seriously though, you have now got a dedicated stock of ebony that can be used for fretboard markers, headstock logos, neck heel cappings, rosettes etc. There will always be something it can be used for.

Darren

Cut a filler strip from the end of the fretboard that's the right width to fit the slot, (it'll be fragile as heck) and glue it in.

Cut the fingerboard all the way through the misplaced slot, lay it on a sheet of wax paper, and glue the two halves together, then proceed as it were a single piece.

(Or save this for when you've the confidence to fix it and make another fretboard!)

In the case I buy a new fretboard. Do you recommend me a way to "recycle" the first fretboard. Can I use it for another purpose ?

@mario So you are buying a new fretboard, that’s fine. 👍 

Definitely save the old fretboard, it can be used for many things in the future, just store it away somewhere for now. 👍

I meant to set the veneer in loose but gluing would probably be better, leaving a small gap at the ends to fill with dust and glue. This can be made to look invisible with care. I would make it the same width as a fret so that the fret can be glued in.

Have you tried to glue directly the two pieces of fretboard and then cut the fret slot in the join (without using veener) ?

This looks a better idea if you glued with ordinary wood glue and used plastic or sticky tape as backing. Just be careful in the fret slotting jig.