I'm not quite sure this topic fits entirely here under bugs, but it is the closest I could come to an appropriate category.
The problem is that if the defender in coli switches his defense while you are setting up your attack, you attack the new unseen defensive setup.
As far as I know It has always worked like this and I remember it being a problem several months ago - but it has not been an issue for a while now - I guess there has been sort of an unspoken gentleman agreement not to do this, as it sucks the fun right out of the game and is a display of poor sportmanship.
But recently certain people have begun exploting this flaw at reward time once again - switching around their lines repeatedly after taking first place - thereby causing a high chance of attacking them blind.
I assume this is not intended.
Ideally people should be able to switch their defense as they please, so what I propose is that the def is locked for the attacker as soon as the setup for the attack has started, so you are able to actually see what you are up against. Otherwise the outcome becomes far too random - eliminating tactics in an otherwise entertaining and important part of the game.
Another, less desirable option, would be making first place unable to switch defense lines.