I've seen it gets tripped up if you mix something common with something uncommon. ChatGPT chokes on, for example:
"Two trains on separate tracks, 30 miles from each other are approaching each other, each at a speed of 10 mph. How long before they crash into each other?"
"Two trains on separate tracks, 30 miles from each other are approaching each other, each at a speed of 10 mph. How long before they crash into each other?"