To reproduce:
1. Install 4.3f4 Unity version on 4.2.2f4 (to the same folder)
2. Set the default OS script editor to SublimeText
3. Create a new project with a freshly installed Unity version
4. Verify that "Unity -> Preferences -> External Tools -> External Script Editor" is set to "MonoDevelop (built-in)"
4. Create a C# script by right-clicking in the project view "Create -> C# Script"
5. Double click on the script
6. Notice that SublimeText opens the script instead of MonoDevelop

Workaround:
1. Unity -> Preferences -> External Tools
2. Click on the "External Script Editor" setting
3. Click "Browse"
4. Navigate manually to the freshly installed MonoDevelop app (that came with the newest Unity version)