TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.

(what the letters from the IRS mean)

I GOT A LETTER FROM THE IRS   

https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs

I AM BEING AUDITED

If you purchased Audit Defense:

https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited

AUDIT SUPPORT

TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support.   At the page below, click on the blue button that says "Get Help from TurboTax Support".  Then it will ask you what tax year is your letter.  Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.

https://support.turbotax.intuit.com/irs-notice/audit-support/

Did the letter from California say why or what they changed on your return?  

I would check over your return and see if you can see anything wrong especially in the payments section.  Make sure you agree with all the amounts listed.   Like is the difference the same or close to the refund you got last year?  Maybe you put down that you applied last year's refund to this year?  Or you put down you made a payment with an extension or paid an estimated payment that you didn't make?

They're claiming I underpaid what was owed even though TurboTax used the tax table to determine how much I should've paid out during the year.  That was the only thing different from my/TurboTax's calculations vs. their "revised" calculations.  I've printed a copy of my return and will be getting a copy of the tax table and calling the FTB to see how they came up with their revised calculations.  They're saying I underpaid $990 exactly.  I've never owed anywhere near that much even when the bulk of my income was from 1099s - I'm positive they've made a mistake and am contesting their findings.

I know they wouldn't know about the letter I received; I was wondering if anyone else in CA had ever filed with TurboTax and then received a letter from the state saying they'd grossly underpaid their taxes.  

I have not seen other posts with the same CA issue ... however CA is notorious for sending erroneous notices.