This database was originally a SQLCE, then migrated to SQL Server, so that might explain the odd seed value, but that doesn't explain the add sequence. It is still happening:

14423 2014-06-26 20:40:15.853
13423 2014-06-25 21:36:50.967
12423 2014-06-24 10:03:07.233
11425 2014-06-23 04:08:36.647
11424 2014-06-22 17:34:20.477
11423 2014-06-22 17:19:30.600
10423 2014-06-21 17:22:57.583
9423 2014-06-19 22:00:17.297
9422 2014-06-15 13:08:41.120
9421 2014-06-14 19:34:37.860
9420 2014-06-13 08:41:19.453
8420 2014-06-13 06:59:05.763

and SELECT IDENT_CURRENT( 'mp_Users' ) returns 15422 showing that another thousand failed inserts have taken place since the last record was added. I say failed inserts because those would increment the identity value, but so would creating and deleting a row of course. I'm guessing something is regularly trying to create accounts in my site, and gives up after 999 attempts. However the site shows no other signs of being under attack (no spikes in Google Analytics etc).

After adding a new user manually, this goes to 15423 as expected. I cannot make a failed insert through the registration UI - validation appears to prevent attempts to write to the DB, as it should. However I don't see what else could be causing this. I'm not sure whether/how to add a trace to the database hosted on Arvixe?  I'll look into this, but any tips appreciated.

There's no dev environment connecting to the DB on Arvixe. I've had a quick look in IIS logs and there are a lot of hits on register.aspx from one i.p. that could possibly correspond to the problem, but it's not conclusive as there are lots of log files with only a handful of hits on this page.  I need to load the logs into a database and get forensic, if I can find time.

I am also wondering whether it could be something to do with the Janrain social sign-in, which is enabled on this site, but there are not huge numbers of hits on OpenIdRpxHandler.aspx (which I assume is used for this?). What should I look for?

I've had a very quick scan of the mojo code for things that might be re-trying 999 times against the mp_Users table, but not spotted anything in the User business areas. DBSiteUser.UpdateUser gets called a lot more than I expected during registration, but that is obviously not the problem!. In passing though I spotted some other things that may possibly be worth noting - perhaps bugs:

• DateOfBirth is not present in the public properties of SiteUser class (SiteUser.cs).
• On creating a user, if the entered DateOfBirth does not parse as a date in the CurrentCulture, it is instead added to the mp_UserProperties table. If the value does parse, it is added to mp_Users.DateOfBirth, and an empty value is added to the mp_UserProperties table. That doesn't seem quite right?
• The calendar control uses the profile settings datePickerYearRange="c-100:c+0" to control what years are available. However on opening the calendar the actual range is calculated from the value in the control, if one is present. This means that the years available change depending on what already entered. Presumably it should be relative to the current date, not the value in the control. I would like to have it work for users between 16 and 80 years old, for example, and this goes crazy if we set the range to ="c-80:c-16" because the year entered then cannot exist in the calendar when it is re-opened.

I'll post more if I find the cause of the UserID gaps.