Unity is unable to open the built-in Monodevelop editor by default and open the system editor instead:

EditorHelper: Failed to open file: unable to get process descriptor for
/Applications/Unity/MonoDevelop.app.
EditorHelper: Unable to use the internal script editor
/Applications/Unity/MonoDevelop.app. Trying the default system editor now.

(from editor.log, 4.5.4p3)

Repro Steps (only reproducible on user's machine):
1. When I double click a CS file in the Unity project the correct solutions file isn't loaded, MonoDevelop will load the previously loaded solutions file.
2. When I double click a line in the Console the correct file is opened in MonoDevelop, but MonoDevelop doesn't automatically go to the correct line.