The Senate GOP tax plan unveiled last week includes a provision that would tax stock options and restricted stock units at vesting, rather than when they are exercised (as under current law) or when there actually is a liquid market (as under the amended House proposal).

Why it matters, per Greg Grogan, a compensation and employee benefits attorney with Simpson Thacher & Bartlett: "Employees will owe taxes on 'potential' value on the vesting date of their options before they know whether the option will ever really be worth anything and before there is any cash available to pay taxes."