When I was looking at Issue #256, I decided to test what would happen if I planned a duration mission and did not set a duration. So I selected Manned Orbital Duration (mission 25), spun the duration pie around to the Unmanned setting, and clicked "Continue." The game didn't see a problem with this, and allowed me to select a capsule & crew as normal. Next turn, I handled vehicle assembly and set a launch date.

The game will allow me to fly the mission, even though it reads "No Duration." Unfortunately, I don't have an advanced enough save game file so the mission penalty caused me to fail too many steps too frequently to see what would happen.