If you add an IPI code to an artist with others IPI adds pending, then the your add fails. Example:

DATE & TIME	Edit #18715891	Edit #18715935	Edit #18716025
2012-08-15 08:38:43 UTC	Opened: add IPI=00217143597
2012-08-15 08:54:31 UTC		Opened: add IPI=00018832481
2012-08-15 09:09:25 UTC			Opened: add IPI=00002052553
2012-08-29 09:00:23 UTC	Applied
2012-08-29 09:00:24 UTC		Failed dependency
2012-08-29 10:00:08 UTC			Failed dependency

The failed edits have the ModBot note:

IPI codes have changed since this edit was created, and now conflict with changes made in this edit.

so I think my adds was probably changes of an empty IPI field, or have something to do and interact with others IPI edits related to the same artist.

I don't know what kind of problem it is, but looking at the user interface, for artists without stored IPI codes, we can see (example):

IPI codes:
	Add IPI code

but it must be only:

Add IPI code

and the add should not affects any IPI entry pending.

The interaction between the various IPI codes by the same artist can also be seen from the example of an artist with two IPI codes, one of which had to be changed:

Edit made only on an IPI row:

Result regarding the two IPI codes:

Looking at the 20120412-add-ipi-tables.sql update to support multiple IPI codes for artists and labels (MBS-2532) I see every single IPI code is stored in a record, and so a single edit related to a single IPI code should affect only that row (artist, ipi) in the artist_ipi table and no other row or table. Isn't it?