If you look, when Anakin has Obiwan bent over and is choking him out using his cybernetic arm, Anakin and Obiwan are holding their respective sabers.
The scene cuts away, and then back to Anakin bending him over, but Anakin is holding Obi-wan's saber, and Obi is holding Anakin's.
Apparently, this happened A LOT in Episode 3 where people picked up the wrong sabers and it wasn't caught.
Apparently, if memory serves me correctly, there was a cut in the scene and when the action recommensed, they picked up the wrong sabers accidentally.