Memo from the State Department: You Need a Current Passport to Go to Mexico

Or at least proof you have applied to renew your passport. Which you can't get until 7-10 days after you apply. Which is really inconvenient when you discover this 3 days before you need to leave for Mexico. So you might have to fly to Colorado, which is the only passport office accepting appointments. Or maybe, if you're lucky, you can pay someone a whole bunch of money to handle it for you. But you won't know until morning how it's going to work out.