In my current career, just a few SCANsat probes in polar orbits around Kerbin, Mun and Minmus. My previous careers are another story. The last career had a mothership with two landers and two probes attached in orbit around Duna that fell victim to the dreaded docking bug, ten frozen Kerbals waiting for me to get up the guts to edit the save file and manually fix the ports. The loss of that mission was one of the driving forces behind starting my current career, but I probably put in another fifty or sixty hours cleaning up some other loose ends before pulling the plug.
In a career using KSPI-E and OPM, I grossly overestimated how well my microwave transmission constellation would work over interplanetary distances and thought that the solar panels were magic. I unlocked the warp drive, sent Val out to Plock as my first test, and had no means of coming home. She sat out there, all alone, waiting patiently for me to send a rescue mission that never came. I still have the save file, so she still has a glimmer of hope, but I know in my heart that that particular universe's Val is destined to float in inky darkness forever.