Hey Trigger, So after more experimentation, I don't think the problem is actually caused by Kerbal Alarm Clock. Tonight (with no Kerbal Alarm Clock installed) I spent some time making my first SSTO in career mode (or trying anyway). When I tried playing around with it, I essentially got a game freeze whenever the flight ended or I tried to revert. I need to play around with some things clearly - uninstall all the mods, try with a different save file, etc. Point is, the behavior seems to be a bit sporadic and it was probably just coincidence that everything appeared to be working after I disabled Kerbal Alarm clock. I'll come back with more data if needed tomorrow or so, but for now, false alarm (as far as the alarm clock is concerned anyway).