I've faced with one play situation in local Russian indoor tournament that slightly confused me and I want to realize how this situation should be treated according the WFDF ultimate rules.
And how about USAU?
That appears to be a foul by the defender in blue.
12.8. Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position.
Initiate contact: Any movement towards a legally positioned opponent (either their stationary position, or their expected position based on their established speed and direction), that results in unavoidable non-incidental contact.
12.9. All players must attempt to avoid contact with other players, and there is no situation where a player may justify initiating contact. “Making a play for the disc” is not a valid excuse for initiating contact with other players.
17.2.1. A Defensive Receiving Foul occurs when a defender initiates contact with a receiver before, or during, an attempt to catch the disc.
The player in blue alters their path to move in the way of the player in red, who has a clearly established speed and direction. It would not have been possible for the player in red to avoid the opponent.
NOTE: Had the blue player not altered his path, but also been moving in an established speed and direction, then it would have been an offsetting foul:
17.10. Offsetting Fouls:
17.10.1. If fouls are called by offensive and defensive players on the same play, the disc returns to the thrower.
17.10.2. Non incidental contact that occurs as two or more opposing players move towards a single point simultaneously should be treated as offsetting fouls.