The first corner case allowing overactivation is written into the rules: if your opponent has no activations remaining you then activate all of your remaining creatures. This is what I term Simple Overactivation. The second corner is the result of the initiative rule: if you activated in the last phase in a round and then in the next turn you have the first phase then you will activate three or more creatures before your opponent will get to react. This will be called Initiative Overactivation, and if a creature is activated twice that will be termed Double Activation.
Initiative overactivation is the more powerful of the two overactivations. By activating last and then first you have the option of double activating a very powerful creature to maximum effect. The classic example is double activating a Large Red Dragon. You can move the dragon 3 times it's speed and then use it's breath weapon. With a flying range of 24 spaces you can move all the way across the short end of the battle grid with movement to spare. And with that range you can easily use it at the boundary of an early game turn to attack a clump of your opponent's warband.
Initiative overactivation does not really provide any intrinsic advantage over simple overactivation other than allowing double activation. Simple overactivation is a more reliable form of overactivation that does not depend on you winning the initiative but instead relies on you outnumbering your opponent. The effects can sometimes be just as powerful, especially if you have saved up all of your "power hitters" for a concentrated attack on a valuable figure of your opponents. Extra consecutive activations can allow you to respond more effectively to an early event like destroying the key guardian protecting the fragile elements of the warband, so the other creatures can move on and go in base to base contact with a spell caster or destroy a key commander. And the more creatures you have to overactivate with generally the better.
It is primarily because of this outnumbered activation that some warbands have been bloating in size. In addition to creating an opportunity to gain overactivation a large warband also serves as a very powerful defense against being overactivated against. Given these two factors feeding it there sometimes is little argument against using 5 Men-at-Arms instead of a Warforged Fighter. While comparable in attack and durability (actually an edge to the war forged in durability with all its immunities) the biggest advantage you get with the tin can brigade is two whole phases dedicated to deciding if now is the right time for your LG beatstick to strike or not.
Many warband builds specifically do this as a feature: Forward Turtle (a Large Silver Dragon band with 11 activations), Malice HEBI (Two Half Elf Bow Initiates with 4 Elf Warriors) most of the Trifecta builds, and the Ryldfecta builds all have large numbers of creatures with full intent of using the ability to time the best attack and use overactivation if possible. Interestingly enough, there is even a command figure in CG, the Moon Elf Fighter, that can give (to a limited degree) simple overactivation at any time.
This, in my fallible wisdom, is why 200pt skirmish bands generally need at least 8 figures to be competitive. Next we will see why this is causing games to be longer than they should.