The great monkeywrench to throw into your meticulously planned actions in HGB has to be an Airstrike. Airstrikes, for those that don't know, are a Support Point (SP) option that can be purchased to help the ground-based forces of the game. Airstrikes are deadly because when they attack, they only count terrain within 4" of their target and are armed with such things as AGMs. Chances are their target will be some sort of stationary unit you have that has a large gun which has been firing with impunity from beyond a piece of terrain blocking enemy line of sight. In other words, your BFG has suddenly been taken out by something that costs no TV and you have to change plans in a hurry. In a game as quick as HGB, that deciion can win or lose the game for you!
To counter Airstrikes you have the option of purchasing Anti-Air Assets. I found this turret from the old range of packaging when they had one to a blister down at The Hobby Kingdom in Burlington. Here's the results of me licking some paint on it:
For a single SP, I get to shoot back at those pesky Airstrikes. Airstrikes, I should also mention, can only be fired upon by those weapons with the AA trait. Most forces in the game really only have 1 or two units that have a dedicated AA weapon. For the South, the Hun and Visigoth have AA weapons, while the North has the Aller. If you don't want to sink a huge amount of TV into an MBT, this is a viable option. The one fault of an Airstrike is their fragile nature: they only have two damage boxes and the armour rating of a mainline Gear such as a Hunter or Jager, so getting a bead on them will definitely help counter your oppenent's monkeywreches with one of your own!