When you change perspectives from first to second to third person while riding a vehicle, it will make you're arm appear turned slightly inward. It changes back to normal when you exit the vehicle and change perspectives once again.
MC-82489
Graphical Glitch - Player Arms
Resolved
Duplicate
0
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-1349. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.