I work at the busiest Walmart Pharmacy in Canada - we've been #1 in prescription orders and #2 in overall sales for 2009 and 2010. Many of these rages come from working there:
1. Slow... old... people. Whether it's walking in the aisles, talking to me on the phone, in person, driving, etc. if I need to get somewhere or we're busy, get to the point as quickly as possible and move on. Just because you're retired and have all the time in the world doesn't mean everyone else does.
2. At work we have an easy ordering service where you can punch in your prescription number over the phone without speaking to someone. Many times I'll receive a phone call and it's someone who doesn't want to use the automated system or doesn't know how to and they give me their prescription numbers. If it's legitimate reason for calling in then that's doesn't bother me, but a significant chunk of these phone calls would be eliminated if people would just use automated ordering. Coincidentally, it's mostly old people who don't use this system. Refer to Rage 1.
3. Someone already mentioned welfare/crackheads in here. Without getting into detail, I'll second that.
4. It's hard to put this into words, but I've always been a super nice, super caring, heart of gold type of person. I feel like working in the pharmacy has ruined that to a degree because I bust my ass and some customers still get pissed. Me me me gimme gimme gimme I-don't-care-how-busy-you-are-the-world-revolves-around-me-I-want-my-meds-now. It's funny in that it's usually customers who have gobs of free time (welfare, old people) who are the most pushy. I wish I could just tell a customer "getting pissed and angry won't get you better service. Calm down, let's have a civilized conversation and both of us we'll come to a solution with smiles on our faces."