Tested with GZDoom 4.7.1 and g4.8pre-310-g1c4d48f5f
In short, the iterator will sometimes fail to detect lines that are right next to the (actor's) position where the iterator is created.
This PK3 has a replacement MAP01, but really, you can test this with any map. It also has a event handler that creates the line iterator where ever player 1 is with a search radius of 64. It prints the index of each found line and spawns a visual indicator in the middle of the line.
In the test map, for example, no matter how close I get to it the iterator will never find linedef 11.
And right at the start, you can see it detects line 6 twice.