When using the advanced track editor to change a track number, it doesn't appear to ensure that all tracks are uniquely numbered. So if you have tracks 1-11, and you type a "7" in the track number field for track 1, it will be accepted even though there are two "track 7"s.

The end result is somewhat expected (It moves track 1 so that it's after track 6), but could have some unintended effects, which probably get complicated (what happens if you set all tracks as "track 1"? — answer: it entirely reverses the track listing)