A mistake i do too often is to forget to modify/lower a lot the range of recon troops and any squad i am trying to infiltrate in a place.
Nothing better than having your soldiers to open fire on their own in your opponent turn at 500m, missing obviously and by doing this giving away their position so your intent of a backside sneaky strategy is totally destroyed.
I like sniper teams. I like to place two snipers in a single hex, but sometimes I click, and the sniper does anything to avoid buddying up with his fellow sniper. Into the road he goes, and I grumble under my breath waiting for the sign "marksman destroyed".