I'm thinking its not that big of a deal

Backround: I'm at the store doing some grocery shopping. There is a mom there with two little kids. One pretty small. We are going up and down the same aisles, but in opposite directions, so pass each other every now and then. The little one is staring to cry and she says something to it about getting tired of shopping or something. All in all pretty good kids; but obviously not having that fun of a time.

Anyway - I get all done and head up front to check out. There are about 4 cashiers, but I notice only two have lights on (meaning that the other two are going on break or whatever). I notice the mom is standing in the line with the light off, and figure she must have been the last person in line before the light went off. I look at the two available cashiers and pick the one with the hopefully shortest line. I'm about two checkout stands away from the mom.

Cashier: finishes with a customer, starts to checkout the next one, the one in front of the mom, and looks up noticing the mom. Excuse me mame, but I'm closed.
Mom: in a very irritated and loud voice. Well you could have told me that before I spent all this time waiting in line! by my accounts, all of 3 minutes or so
C: Oh, I'm sorry. I'll go ahead and take you.
M: NO. THATS OK. and turns and moves to another line
...
M: now on the other side of me, and mumbling to self. She picks her kids up out of the cart If there is one thing I hate... come on

AND THEY LEAVE THE STORE.

Lady behind me: What was she so upset about?
Me: The cashiers light was off and she got in line and the cashier told her she was closed so she went to the other line.
Lady: Yeah I know... but I mean, what was she so mad about?
Me: That was it!
Lady: Just looks at me and shakes her head Oh...
Me: Yeah, some folks. Doesn't take much