Learned that one the hard way too, but it wasn't the string line that lied to you. A line only reads level corner to corner, it can't see a dip in the middle of the footing. You can have both corners dead on and still have a low spot right in the middle, and the line just bridges right over it like nothing's wrong. That's why the lean showed up around the fourth course, the wall finally piled up enough weight to follow the dip. So yeah, trust the line for straight, but check the footing itself with a level before you ever lay the first course. After that spot checking every few courses is fine as long as your bed joints are even.