This was recently pointed out by DeeDarFan, doesn't work with the current version. Surprisingly, this only work with HD Edition pre-4.4.

• Open Scenario Editor
• Units > Select any unit.
• While it's selecting, quit Scenario Editor.
• Now select the mission you want, it'll get the Dark Age glitch. Time of activating it counts against your time, be warned. Passive effect: With some maps contain bridges, they will become impassable for some reasons.