at a connection point, you may be an A group to board but find that there are already a number of people sitting on the flight who have normally all moved to the front rows and exit rows after the other people got off.

If you pay for upgraded boarding in the hopes you're going to get an exit row, make sure that there are no people already on board and that you're no higher than an A3. Sometimes one person in a group will pay the additional fee to board as a low A, then get on the plane and box out the exit row for their friends that are stuck in the C group. I think it's total BS, but my wife says it's not worth the scene I'm going to make inside a sealed tube with that person for the next X hours.

Also, if you're starting at a connection point and other passengers have checked in for their original leg AND your same connecting flight, they will have checked in for both flights hours before you're even allowed to. Dozens of times I've watched the clock tick over to my boarding time, checked in within mere seconds of the right time and found myself in the high B group.