Right now I'm running 1.2.2 and 1.2.1 alongside each other. What I've noticed is that craft built or modified in 1.2.2 won't load in 1.2.1 because it says they're not compatible. A simple edit in the top of the craft file -changing the version to 1.2.1- makes it work, at least with my rover.
Now I wonder if the version checker should be so rigid, especially between minor updates like this one.
I can imagine a lot of people might encounter such things on KerbalX, @katateochi. It happened to me at least once that a user came to me with a question.