People who carry only $20 bills and need to pay $2. I was at the theatre today and a theatre patron who knew he was getting a hearing assistive device which costs $2 and gave a $20. At 5 minutes to showtime, there was a lineup of 10 people behind him and the staff had $2 and the right bills in change but not the final $1 to make change. She offered to reimburse him at intermission but he insisted that she find the extra $1 from the box office until those of us who had pre-ordered the device and were waiting to pick them up started grumbling loudly enough for him to hear. Finally he said 'OK, I'll come back".
Keep a few coins handy!