ESRI did not make mxds saved in version 10.3 compatible with earlier versions of ArcMap. Be careful when saving basemap mxds since other users are still on 10.2 .
To save a mxd in 10.3:
File -> Save a copy
Save as type: ArcMap 10.1 and ArcMap 10.2 documents
Original report by AndyB (Bitbucket: AndyB, GitHub: AndyB).
ESRI did not make mxds saved in version 10.3 compatible with earlier versions of ArcMap. Be careful when saving basemap mxds since other users are still on 10.2 .
To save a mxd in 10.3: