We have table with an identity integer primary key, and a single procedure inserts to it. This has worked for years and has inserted 100's of millions of rows without problem. A few months ago we upgraded to SQL 2012, but this code and this table did not change. Today we were alerted to insert failure, PK violation, error specified the key that was problem. Such a row already existed. This has only happened once.
We manually verified that the current identity seed is correct and continues to increase as expected. We've run DBCC CHECKIDENT (table, NO_RESEED), came back clean.
The only possibly theory I can come up with is that there is a bug that allows this to happen very rarely--two simultaneous connections get same identity value, one fails.