Entrepreneurs and investors in California can expect to receive a rude shock in the mail if they sold their company in the last four years. Not only did the state's Franchise Tax Board (FTB) eliminate a tax break on capital gains for small business owners and investors, it announced the tax would be reinstated retroactively. This means those who benefitted from the break can expect a bill for unpaid taxes, plus interest, stretching all the way back to 2008.

Since 1993, California entrepreneurs and early-stage investors have enjoyed a partial state income tax exclusion on sales of stock of a "qualified" small business. This was an incentive for people to start and keep businesses in California. If they sold their company, they would only have to pay half of the regular state tax rate on what they gained -- about 4.5% instead of 9%. That could include founders of companies such as Instagram and Yelp (YELP).

The FTB announced its decision last December, and the ruling went into effect earlier this year. Now, not only will stockholders have to pay the full tax rate on capital gains, which has risen to about 13%, but they'll also be billed retroactively for 50% of the taxes they excluded. The FTB says this will affect over 2,500 people and bring in about $120 million in revenue.