- I saw a man in the room.
- I saw the man in the room.
1a. I saw a man who was in the room. There were five people in the room, for example. I saw one man who was in the room. In this case, 'in the room' can modify 'a man'.
1b. In the room, I saw a man. In this case 'in the room' modifies 'saw' in Sentence 1. So Sentence 1 also has structural ambiguity, doesn't it?