Identity column value falling behind randomly
Randomly, one of these tables' identity values will fall behind, stopping any inserts from happening and we have no idea why.


Inserts are probably stopping because of an attempt to reuse an already existing unique value in the PRIMARY KEY, thus triggering the error like:

Msg 2627, Level 14, State 1, Line 27
Violation of PRIMARY KEY constraint 'PK__ID__1234'. Cannot insert duplicate key in object 'dbo.MyTable'. The duplicate key value is (8).
The statement has been terminated.


Note that if your IDENTITY column does not have a UNIQUE index or constraint, it is possible to reseed repeatedly and have many identical ID values. You do not want to do that, of course.

I have not personally found an error that, in itself, would reseed the IDENTITY value.  Of course, it is possible to reset the SEED to a range where there will soon be a conflict by running a reseed that is lower than the current seed:

DBCC CHECKIDENT( MyTable,RESEED, 7) WITH NO_INFOMSGS 


It could be that code somewhere in one of the processes actually does a RESEED on the table under some unusual circumstances. 

(For example, this could be from a merge of two data sets, where the code reads the high value from one data set and after the import RESEEDs to the lower of the two high values that were merged.)

You should also read Martin Smith's post at: https://stackoverflow.com/questions/14146148/identity-increment-is-jumping-in-sql-server-database