I continually have the wrong hunch about what's in and what's out in ultimate. The league rules (https://www.vul.ca/sites/default/files/files/USAU11thEd.pdf, section IX, C) state:
C. A player contacting the out-of-bounds area is out-of-bounds.
C1. If momentum carries a player out-of-bounds after landing in-bounds with possession of an in-bounds disc, the player is considered in-bounds. For this exception to apply, that player’s first point of ground contact with any area must be completely in-bounds.
C2. A pivoting thrower may contact an out-of-bounds area, provided that part of the pivot remains in contact with the playing field.
What this means is that a player who is visibly out-of-bounds (C) may actually be in-bounds in certain situations (C1 and C2). This leads to a great amount of subjectivity in calls (arguments about which foot touched first, whether you actually pivoted in or out, etc), and confusion for new players who are unfamiliar with the exceptions.
Other field sports like soccer and football, and other bounded sports like basketball, have much clearer rules. Out is out. You are not allowed to play the ball when it is out, and you cannot step out while making a play. This is easier to understand and enforce than the variations on what is considered in-play in ultimate.
C1 is more understandable, as running out after catching a disc is often unavoidable. However, C2 seems like an unnecessary complication of what would otherwise be a simple rule. Imagine a football quarterback who could step one foot out of bounds and make a throw, or a basketball player who could step one foot off the court and make a shot. I'm not sure what we gain by allowing it.
I know it's not likely to change, but it's one part of the rules that I find illogical and hard to apply on the field.