Double-trigger RSUs need two things to happen before they are yours. The first trigger is the usual time-based schedule: you have been there long enough for a block to vest. The second is a liquidity event, most often an IPO or an acquisition. Until both have happened, the shares are not delivered and not taxed.
Private companies use this structure to avoid the problem of single-trigger RSUs, where employees owe income tax on shares they cannot sell. With double trigger, nothing is taxable until there is a market for the stock.
The planning consequence is that the tax arrives in a pile. Suppose you have worked at a startup for three years and $300,000 of RSUs have met the time trigger. The company goes public, the second trigger fires, and all $300,000 becomes ordinary income in one year. Withholding at the 22% supplemental rate (2026) will fall well short of the 35% or 37% you may actually owe. Many people at newly public companies face a surprise tax bill the following April for exactly this reason.
The common mistake is leaving before the liquidity event. Under most plans, RSUs that met the time trigger but not the second one are forfeited when you leave, though some plans keep them alive for a period. Read the plan document; the answer decides how much your equity is worth if you resign.