This has happened in a Normal and a Hard difficulty game. When the invasion fleets in the NW that are spawned by achieving a long victory, they immediatly attack that island fortress Bornais. If you decide to play out the battle instead of just letting them auto-wipe the garrison, they refuse to land their boats at the slots inside the city walls... and they brought no siege equipment so they end up just shuffling around outside the walls, getting shot at by towers. You just have to let the clock run out to get a draw. Usually three to four of their armies do the assault so you have to repeat the process a few times.
Their inability to land inside the walls makes the city unconquerable. Yeah, I could be realistic and just let them auto-wipe the garrison, but bad AI should be punished.