That person in front of you at Subway (usually a kid or very indecisive adult) who takes 5 minutes to decide on each option they're given.
I once walked in behind a guy with three kids under 10 and walked straight back out, went to another store for 30 minutes and came back in time to see the receipt being handed over. Leaving and coming back was easily in the top 10,000 best decisions I've made in my life.
If you're approaching a Subway and some guy also heading towards the door breaks into a run to beat you to it, it could well be me. You never know how long the customer in front will take so it's better to be safe than sorry sometimes!!!