My most pleasant surprises were two vehicles that my brother and I received at the same time back in '86: I remember that my brother and I bought the Stun one day at Plaza Cote-Des-Neiges' Wise store here in Montreal Quebec, and the next day we brought it to school to show off of course (hey it was our first vehicle), and one of my friends who saw it told me that he received as a gift a Havoc and didn't like it. We traded a few Star Wars figures for it and that's how we got the Havoc another day later. My brother and I just didn't have enough dollars back then in our piggy bank to get more armada for our Joes and Cobras, but I remember loving these as they were not too big and had an interesting figure capacity and multiple playing features.
I remember that we pretended that the Havoc was the Joe's mobile base, and that the hovering scout vehicle was our Skyhawk. Kind of the same thing for the stun, with the obvious Cobra insignias on the wheels and the banners, we liked to imagine that it was our mobile Terror base and that the two cannons at the front had the power of two rotating thunderclaps.
But....when I started collecting GI JOE again, that's 10 years ago, I remember buying a vehicle lot, and when I saw the Mean Dog I was "what is that, oh well, it says GIJOE on it". I dropped it by accident and thought I had broken it when the two halves seperated. But Man, I got the surprise: the vehicle could actually split in three pieces! That was perhaps my best discovery, and disappointment when I tried finding this feature in other vehicles, but found it only in a few like the Maggot per instance.