There seems to be no "unwritten rule" for . . . .

this situation:

You are in a supermarket or department store.

You take yourself and the two items you intend purchasing to the end of the long checkout queue.

12 people are ahead of you and the adjacent checkout is closed.

You eventually become the third, or even second person in line.

The checkout employee declares that the checkout is closed, due to cash or technical difficulties.

The adjacent checkout isle is opened.

The line of 6 or 7 people that queued up behind you suddenly swerve across to the new checkout in reverse sequence and they are now in front of you - due to the fact that a stainless steel rail prevents you from sliding across.

Don't you just feel ripped off?!