Jeb had a bad landing on Duna, and lost his stage that would take him to Duna orbit. Luckily the interplanetary transfer stage had just enough fuel to get to orbit and home. No fuel left just barely able to get into Kerbin's atmosphere.
For me it's sending Jeb to Jool tried aerobraking went to low and hit Jool then finding out I did not quick-save. (You can see Jeb's last photo to the left.)
I leave my rover on one of Kerbin's moons go do something else. Then I go to the tracking station to pilot my rover, but it's gone. I can't figure out what happened.