Basically, None.

Follow the instructions, take your time, finish the process making sure the Maestro bottoms out on every fret.

If you do not finish you will think, "oh my god, I ruined my guitar", because it will not play or sound right, might even have some good buzz.

You did not ruin your guitar, you simply did not follow through with the Maestro.
Go back and follow through, everything will be perfect.

FretMaestro cannot ruin your guitar except:

1. User Error: You must make sure that the dial settings match, 3 with 3, 5 with 5.
   If not the file will be uneven, and that is not good.
2. User Error: Torquing the file sideways while filing, as is true with all files, this makes the file cut the sides of the frets.

Yes, and no. For heavy leveling see our tutorial...

How To

Fretboards that have been over humidified or dried out will not be true. They've either been shrunk by dry out, or expanded by too much humidity. This must be fixed before any fret leveling work can correctly be done.

Use a notched straight edge, adjust the truss rod. If the neck will not flatten using the truss rod adjustment, this needs to be handled before leveling frets.

Asymmetrical fretboards are the result of poor care.

Humidity or arid conditions over a prolonged period of time can and will cause the wood to expand or shrink. This is not a fret level issue, it is a fretboard/neck repair or replace issue

The other and most common problem is that someone misused the truss rod, this happens when adjusting the rod tighter while under string tension.

In such cases. I recommend either replace the neck, repairing it. This should be done before doing any fret level work.

Yes... however, in case of having to remove a lot of fret material to get rid of divots or to straighten out a prior wonky fret level, we show a new way to use a sanding beam to rough in the level, and then finish with FretMaestro.

No.