When running shakedown I usually open 2 browser tabs side by side, with 2WD in one and 4WD in another (in order to look at the already found good setup of the other car).
The problem is that if I opened 2WD tab first, then opened 4WD tab next to it and tried to run 2WD shakedown, it actually runs 4WD for me and as a result I have both tabs with 4WD afterwards. More interesting is the fact that it allows such run even though there is no Traction balance value at all (for a particular example see Hackers RC team, Maori rally, 4WD shakedown run No. 4).
This could be easily fixed by using the car selection from POST form rather than session.