What causes broken craft files?

[Pax Kerbana]

I was working in the SPH yesterday on a set of trucks for the Dres mining base when something broke. Because the two trucks were similar in design and needed matching heights, I had been loading the first truck via the new "merge" button on the load craft menu in previous KSP sessions. This time when the truck appeared in the SPH it broke between parts, with one half attaching automatically to the current vehicle and the other half floating on the other end of the SPH but solid like it was the craft I was working on.

Long story short, I eventually cleared the editor and finished work only to find out I couldn't launch it because "craft size unknown" and something about reloading the craft in the SPH so the Kerbal engineers could calculate the size. Well obviously I tried that but it did not work. Reloaded the game, no-go; did it again, still no-go. Tried opening it in the VAB... still no luck.

Angry (I've spent a lot of time on this) I remade the vehicle using the one good truck I had left by saving it as a new craft and reworking it. But I still need to do all the testing over again because it's not quite as good as the previous version I had, a few tweaks here and there are needed for stability and handling.

What bothers me is that I had just loaded KSP and had just started working on it again when this happened. It's not like I had the game running for hours. This issue... is it a bug of KSP or something to do with a hardware glitch (such as hd)? I've not encountered this before and I have spent A LOT of hours in the VAB/SPH. Did I do something to trigger this?