Depending on local labour laws this can happen to you even without unlimited PTO. I have a coworker in BC who was denied the ability to exercise their vacation during a three month crunch period - and then the company refused to pay out the hours and disappeared them into the aether - this tends to be legal in a lot of places when labour laws confirm that vacation is at the convenience of the employer (which is totally reasonable) but fail to mandate either vacation carryover or vacation payout - a lot of jurisdictions can allow this sort of grey area and if your company leverages it... Quit Immediately.