problems with an invoice number

["Eamonn Hamilton" <..hidden..>]

Hi,

I look after a 1.2.18 system, the the guy operating it has just called me
with a problem.

It would appear he was posting two invoices, both of which were emailed,
but after checking the second last invoice failed to go into the database.
This left him with invoices numbered

1295
1296
1298

with 1297 missing. He then stepped back in his browser history to the post
screen for 1297, changed the invoice number to 1299 and posted
successfully.

Now, I have questions :

a) has he broken anything by doing this?
b) can the invoice number be changed to match the one previously mailed out?

and, of course,

c) does anybody have any idea why such a thing would happen?

Regards,
Eamonn