Underpayment of CA state tax from 5 years ago.

Hi all,

I just received a mail from CA to say 5 years ago I own $1500 dollars.. I was so busy last month and I didn't see it until now and past the protest deadline. I don't even know who they calculated and why the miss. I'm fine paying it. I always do my own tax, so this is from turboxtax.

• Just wonder why didn't they reject the tax return 5 years ago and only find it out now? Is it possible that they are wrong?
• Does this mean I may own federal tax as well?
• what's the easiest way to trace back what the issue was from 5 years ago?

Thanks all..