I've managed to reproduce this once, so this isn't just a freak incident.
When defeating a Hercules Class Titan (Lieutenant) right as it merges with another one (right up next to the end of the animation), the game spawns the boss-class variant AND ALSO a lower health version of the original Lieutenant class, where it should not
The extra special icing on this cake is the fact that the newly birthed low-rank titan cannot be seen, cannot be targeted, can attack you with the full suite of its abilities, and can generally eat a bag of free-ranged dongs. It can be killed with AoE, however, and it doesn't seem to be at full health.
This does not happen under any other circumstance that I can see, but as said I've had it twice now.