In 3d viewport you can toggle between the camera and old view with '0'. This will restore, orthographic or perspective projection, where either of them was seted before. If you don't toggle with '0' and return with '1' for e.g., it won't restore the projection.

Note, this won't restore the projection, when the camera view is left with the mouse. It's a bit related to bug #20159 tho the basic problem there is something different.

Aurel