Wrong Line

I always pick the wrong line when I'm checking out at a store. It could be the shortest, fastest-moving line but as soon as I get in line something happens that slows me down. Today I went to Sam's Club. I dropped by just before picking up Kennedy from preschool so I had limited time. It was very busy for a weekday. There were probably 4-5 lines open and each line had about four people already. I picked one that seemed to be going fast. I didn't realize the lady in front of me had two packed-full carts. They were filled with breakfast-type foods: grits, eggs, sausage, etc. One cart was filled with 5-6 boxes, not cartons, of eggs. She must run some type of business. Anyway, my clock was ticking and it was minutes before I had to leave or be charged for picking up my daughter late. I thought about changing lines but by then the other lines were long enough that it wouldn't be worth it. I only had three items and was a little annoyed that the lady in front of me didn't let me go first, especially since she was cooing at my fussy toddler just before her turn. I don't know how this happens to me all the time. I'm just not a good line picker. What's the secret?
Luckily my transaction didn't take much time at all and I made it to the preschool just in time. Kennedy wasn't even the last one to be picked up.