Missed Orbit Scenario is a situation that could come up when returning to the Hermes from the surface of Mars. The Ares crew get to orbit in the Mars Ascent Vehicle (MAV). Many things can go wrong with a MAV launch, and one of them is that it could end up in a much lower orbit than intended.
If that happens, they have to sit and wait in the MAV for NASA to remotely control Hermes to come to them. However, Hermes is powered by ion engines, that go at a slow speed and constant acceleration rather than a powerful point-thrust like a chemical rocket, so it will take a long time to get it to them.
To prepare for this possibility, all Ares crews have to practice what they’d do in the MAV during the 3 days it would take to get Hermes to them.
Whilst not the worst thing that could happen, the Missed Orbit Scenario is known to be hated by many of the three Ares crews, including Mark Watney and Karen Rhodes (mechanical engineer for Ares 1) as the MAV is designed for a 23 minute trip and not a three day stay it is very cramped with only six seats, some control panels and no privacy.
All this is mentioned in Mark Watney's 'diary'.