I received two W-2 forms from my employer, the only difference being boxes 15-17: one form reflected my MD income tax and the other reflected my CA income tax. Everything I've read so far indicates I'm only supposed to be taxed for the state where I worked (MD), not CA, but is that true? If so, was it an employer error to tax me for CA?
Yes. You have zero sourced wages to California because the income is sourced to Maryland. Sourced income means that you physically earned the wages in that state. Per the State of California, you are only responsible for state income tax if you performed services in California for wages.
However, you will have to file a nonresident California return since your employer withheld California state income tax from your wages. California will give you a refund of taxes paid and Maryland will tax you on the income. Please contact your employer and inform them to only withhold Maryland tax from now on as you are not responsible to pay California state income tax.
You stated that you received two W-2s. Is one of them corrected? If you're not sure if you have a corrected W-2, take a look at the bottom of your W-2. If it’s corrected, it should say Form W-2c, Corrected Wage and Tax Statement. If it doesn't say this at the bottom, you may have a second type of corrected W-2c that has a box at the top marked corrected. This box does not appear on standard (original) W-2s. If the second W-2 does not appear corrected, then add the boxes 15-17 of the second W-2 to the first W-2 by clicking Add another state when you are inputting your W-2.