Or problems like my current game. Thankfully didn't happen to me, but:
1. Siam liberates Antwerp (from the Ottomans)
2. Antwerp decides that it will not ally with Siam, but with Austria, since their influence is higher. (I don't remember the history, but presumably Austria had a lot of influence before, and it just freezes while Antwerp was being occupied)
3. Austria is at war with Siam
4. Antwerp declares war on Siam (its liberator)