It sometimes works, it sometimes doesn't. You're more likely to be caught if you try to board with the first group or two. Unless the gate agent is a real jerk, or you are, the worst that can happen is that you'll be told to wait for your own group.

In 2026, airlines care significantly more about boarding groups than they did in the past, primarily because boarding efficiency is a major factor in "on-time performance" and aircraft turnaround. Most major carriers like Delta, United, and American use automated gate scanners that will actually beep or display an error message if a passenger tries to board before their assigned group. While a gate agent might let you through if the plane is empty, they are increasingly under pressure to stick to the sequence to prevent "gate lice" (passengers crowding the boarding area) and to ensure that premium cabin and elite status flyers get their guaranteed overhead bin space. If you are in Group 5 and try to board with Group 1, you will likely be politely asked to step aside and wait until your group is called. This enforcement is even stricter on budget airlines where "Priority Boarding" is a paid service; allowing a non-paying guest to board early would devalue the product they are selling to other passengers.