I hadn't noticed this until now, but as pointed out by CelticREI on the ATT, you can't nest APs including situations where you add Array or Magic. I know that this was not always the case because I used to have a build involving someone with Weapons Summoning including a grenade launcher with alternate modes of fire.
The problem is very easy to recreate:
1) Create a new character
2) Add Blast 10 to them and add an AP.
3) You'll notice that the dialogue for the AP has nothing below it allowing for another Alternate Power.
As noted in Steve Kenson's post, Nested APs are legal. Indeed, they were once supported, although I couldn't say how many versions ago.
The problem is very easy to recreate:
1) Create a new character
2) Add Blast 10 to them and add an AP.
3) You'll notice that the dialogue for the AP has nothing below it allowing for another Alternate Power.
As noted in Steve Kenson's post, Nested APs are legal. Indeed, they were once supported, although I couldn't say how many versions ago.