Lets say the fmv in 2015 is $500, and your strike price $100. If they are ISO, in 2015 you own no regular tax on it(thats the Incentivized). In 2017 you owe capital gains on $900.

However, AMT(alternative minimum tax) doesnt recognize ISO. So in 2015, you have to calculate AMT, which $400 counts towards. This is basically a no-deduction(except a high standard deduction) flat tax, you potentially owe 26-35% on that 400. Even though you didnt sell anything - this is where people get screwed.

So lets say you paid $100 in AMT. In 2017, you still owe capital gains tax on the whole $900, but you calculate AMT and claim the difference, up to $100, as a credit - the difference should be >100. You can actually claim this credit every year until previously paid AMT runs out, but most likely the difference wont be sizable enough until you sell.