This usually happens if your option has a strike price lower than the market price at the time they're issued, in which case depending on jurisdiction they difference will usually be taxed as income on exercise. If you're really unlucky about where you are and how the options are arranged, you might even get taxed based on the market price on exercise. E.g. Norway at least used to be that way a while back unless you took specific precautions to get the options issued under an approved scheme.

It's something to be aware of, as it means a lot of people might not be able to afford to exercise the options before they're ready to sell the shares, which means your total gains gets taxed as income most places (as opposed to if you see the company on a nice upwards trajectory and exercise and wait, in which case you may end up paying only capital gains and in some cases depending on where you are might end up paying reduced capital gains rates on the growth from your exercised onwards). The difference between a good and bad options plan can easily be 50%+ tax (e.g. UK, if you're careless you might pay 53.8% - top rate income tax + national insurance + employers national insurance contributions, vs. 10% if your company qualifies for certain incentive schemes for startups and the options plan is structured right..)

It might seem great to have a really low strike price, but if you expect to be in it for the long haul and might contemplate exercising options before you're ready to sell your shares, talk to a tax lawyer before agreeing to options terms.